Pharmacology Made Easy 5.0: Introduction to Pharmacology

Learning Objectives

• Differentiate between generic and brand/trade name medications, prescription and nonprescription medications. 

• Discuss the pharmaceutics, pharmacokinetics, and pharmacodynamics of medications.  

• Identify the expected actions and therapeutic uses of medications. 

• Explain the significance of an adverse drug reaction, tolerance, and toxicity. 

• Discuss medication precautions, contraindications, and interactions.  

• Describe the nurse’s role when providing client instructions on medication.  

Medication Categories

Under each body system are various categories of medications. Medications in the same category usually act in a similar manner and share the same therapeutic effects. They also have many of the same adverse drug reactions, contraindications and precautions, and administration considerations.

Since there are several hundred medications you may someday be responsible for, it is easier to learn medications according to their category, rather than learning each medication individually.

Once you learn about a particular category, next up is learning about some of the medications in that category. The prototype for a category is the medication providers most commonly prescribe. That is the medication the module discusses in greatest detail. Make sure you study the medication administration table for each category’s prototypical medication.

Generic vs. Brand or Trade Name

It is important that you know medication by both the generic name and the brand or trade name. A pharmaceutical company develops a medication and gives it an official name. That becomes the generic name of the medication. Generic names are not capitalized. The brand, or trade, name is a medication’s commercial or proprietary name, which is capitalized and may vary according to the company producing and marketing it. Here is an example of a medication you are probably familiar with: the generic name of this pain-relieving medication is acetaminophen, which is more commonly referred to by its brand name, Tylenol. There may be several different brand names for one generic medication, depending on who manufactures the medication.  

Generic and brand, or trade, name medications have the same chemical composition, and there is no difference in potential for abuse or dependence between the generic or brand name of the same medication. The US Food and Drug Administration (FDA) conducts studies to make sure that each new generic medication has the same therapeutic equivalence as the original brand-name medication. Once the FDA establishes this, the generic form can hit the market. Pending approval from the client’s provider, the generic medication is interchangeable with the brand-name medication.

Prescription vs. Nonprescription

Medications that require prescriptions are “prescription medications.” Other medications are “nonprescription” and are available “over-the-counter.”

The federal government identifies prescription medications as potentially harmful if there is no supervision of administration by a licensed health provider, such as a physician, nurse practitioner, or physician’s assistant. Clients cannot obtain prescription medications without a prescription from one of these providers.

Some examples of prescription medications are antibiotics, or medications to treat infections, such as ceftriaxone. There are antihypertensive medications that treat high blood pressure, such as captopril. There are also diuretics medications that promote urination, such as furosemide, and narcotic analgesics such as fentanyl, which are opiate-based medications for pain relief. 

Nonprescription or over-the-counter medications do not require a prescription because they have a designation from the FDA that says they are relatively safe to take without supervision. You can purchase nonprescription medications at drug stores, grocery stores, and other stores that sell this type of product.

Some examples of nonprescription medications are acetaminophen, also called Tylenol; ibuprofen, also called Advil; diphenhydramine, often sold under the name Benadryl; loratadine, also called Claritin; and cimetidine, also called Tagamet.

Pharmaceutics

Pharmaceutics address how various medication forms affect the dissolution of a medication, the absorption rate of a medication, and, subsequently, its onset of action.

Oral Medications

Many clients take medications by mouth; these are known as oral medications. These medications come in tablet, capsule, powder, or liquid forms. medications that a client takes through the mouth but inhales into the lungs, are usually in powder form. The form of an oral or inhaled medication influences its rate of absorption.

Some oral medications come in forms that delay release of the medication. For example, enteric-coated medications, sometimes abbreviated as EC, dissolve when the medication reaches the intestine. Extended-release medications, sometimes abbreviated as ER, release over a period of time. Other terms for extended-release medications include extended length, abbreviated as XL, sustained release, abbreviated as SR or XR, and sustained action, abbreviated as SA. Some oral medications also come in an immediate-release form, which expedites release of the medication after it is swallowed. Oral disintegrating tablets (ODT) dissolve in the oral cavity.

Tablets that are scored, or marked with a line in the center, may be split. Although tablets may sometimes be crushed if a client is unable to swallow tablets and a liquid form of a medication is not available, capsules and extended- or sustained-release forms of medications should never be crushed.  

Parenteral (Injectable) Medications

You can administer parenteral, or injectable, medications one of three ways: intravenously into a vein, which is commonly abbreviated as IV; subcutaneously into subcutaneous tissue, commonly abbreviated as SUBQ; or intramuscularly into muscle tissue, commonly abbreviated as IM. Parenteral medications typically come in either a liquid form or a dry form that needs liquid added. This is called reconstitution.

The onset and absorption of parenteral medications vary depending on which method you use to give the medication. The absorption of intravenous medications is immediate and complete, resulting in an immediate onset of action. In contrast, the absorption of subcutaneous and intramuscular medications is variable. If the medication is water-soluble and the client has good circulatory blood flow and tissue perfusion, the absorption of the medication is rapid. If the medication either is not water-soluble or is poorly water-soluble, the absorption is slower. Also, if the client has slowed or impaired circulatory blood flow or tissue perfusion, the absorption of the medication is slower. Depending on the solubility of the medication and the client’s circulatory status, the onset of action for a subcutaneous or intramuscular injection varies.

Parenteral Medications

Topical or Transdermal Medications

Topical or transdermal medications are medications that you apply to a client’s skin, eyes, ears, nose, rectum, vagina, or lungs. Topical medications deliver a constant amount of the medication over an extended period of time, have a slower onset of action, and a longer duration of action than oral or parenterally administered medications.

Absorption: is the movement of the drug from the site of administration to the various tissues of the body. For example, oral drugs must absorb through the intestinal wall and enter the portal vein before passing through the liver and into the body's circulation. Since part of the drug may deactivate when it passes through the liver, the actual amount available to the body can be less than what the client ingested orally. This is the first-pass effect. Bioavailability is the amount of the oral drug available after passing through the liver. Parenteral drugs absorbed either directly through a vein or indirectly through muscle or subcutaneous tissue enter the blood and then pass through the liver, thereby bypassing the first-pass effect. Metabolism of drugs primarily takes place in the liver. 

Distribution: is movement of the medication by the circulatory system to its intended site of action. Medications enter the blood after they absorb through the intestinal wall or alveoli in the lungs, or after direct or indirect injection. Areas of the body that have a rich blood supply, such as the liver, kidneys, and heart receive the highest level of the medication. Areas of the body that do not have a rich supply of blood, such as the bones, or areas that have a natural barrier, such as the brain's blood brain barrier, won't receive a very high level of the medication.

Metabolism

Metabolism is the change that occurs in a medication into a more or less potent form of the medication, more soluble form of the medication, or an inactive form of the medication. The liver is responsible for most of the metabolism of medications that occurs in the body.

“Biotransformation” occurs when the structure of the medication is chemically altered during metabolism. The new, or altered, form of the medication is called a “metabolite.” The word metabolite indicates it is a by-product of metabolism.

Excretion: is the elimination of a medication or its metabolites through various parts of the body. For instance, the kidneys eliminate many medications through the urine. The kidneys may even reabsorb some of the medication through the kidneys, allowing it to pass through the body again before excretion actually occurs. Excretion can also occur through the gastrointestinal tract, the skin, or the lungs.

Medication Half-Life

The half-life of a medication is the time it takes for a medication that enters the body to decrease in amount by half. This decrease reflects how quickly and efficiently a medication metabolizes and excretes. You might need to administer medications with a short half-life several times a day. Likewise, you might need to give medications with a long half-life only once a day. Since most medications are metabolized in the liver and excreted in the kidneys, clients who have a decrease in functioning in either of these organs may experience adverse and toxic effects of medications more easily if the provider does not adjust the dosage and frequency of the medication.

Onset, Peak, and Duration

A medication’s onset of action is the amount of time it takes for the medication to demonstrate a therapeutic response. A medication’s peak effect is the time it takes the medication to demonstrate its full therapeutic effect. A medication’s duration of action is the length of time the medication’s therapeutic effect lasts without additional doses.

Peak and Trough Levels of Medication

When you are administering medications that must maintain a therapeutic level in the blood to be effective, it is important for you to know about peak and trough levels. The peak level of a medication is the point in time when the medication is at its highest level in the body. The trough level of a medication is the point in time when the medication is at its lowest level in the body. You can determine peak and trough levels by taking samples of blood from a client throughout the day. Have a laboratory analyze the samples to measure the amount of medication in the client’s blood. The variations that occur in the client’s blood levels help determine whether you are maintaining a therapeutic level throughout the day. An antibiotic is an example of a medication you must maintain at a constant therapeutic level in the blood for it to exert its maximum therapeutic effect.

Pharmacodynamics

Pharmacodynamics are the biochemical changes that occur in the body as a result of taking a medication. Therapeutic effects are the intended effects of the medication. Adverse drug reactions are the unintended effects of the medication.

The effects of medications occur secondary to changes in how cells function, changes in the cellular environment, or changes in the action of enzymes in the body.

Changes in cellular function precipitate a greater or lesser response than what typically occurs. Agonists are medications that bind with a receptor and precipitate a greater-than-typical response. Antagonists are medications that bind with a receptor and either block a response or precipitate a less-than-typical response. Medications that are antagonists exert their effect in either a competitive or noncompetitive manner. Antagonists that compete with an agonist for receptor sites are considered “competitive antagonists.” Antagonists that block an agonist’s access to receptor sites are considered “noncompetitive antagonists.” When you give in high enough doses, noncompetitive antagonists can completely block the typical response caused by an agonist. Competitive antagonists that you give in increasingly high doses will decrease the typical response by an agonist but will rarely completely block it. 

Changes in the cellular environment occur when medications interfere with the structure of a cell, such as the cell wall or one of a cell’s critical processes like replication. For example, penicillin-type antibiotics inhibit cell wall synthesis of certain types of bacteria, resulting in the destruction and death of the bacteria. Sulfa-type antibiotics inhibit the replication of certain types of bacteria by preventing folic acid from helping to make DNA and RNA.

Through a process called “selective medication action,” a medication interacts with specific receptor sites to produce the desired effects. Receptors exist for all molecules that synchronize the body’s physiologic activities, including neurotransmitters and hormones. Medications that are more selective will interact with fewer receptors and have a more limited effect than medications that interact with many receptors, allowing for more widespread bodily reactions.

Expected Pharmacologic Action and Therapeutic Uses 

The action a medication exerts in the body is a medication’s expected pharmacologic action. A provider chooses medications for a client’s treatment plan based on the expected pharmacologic actions of those medications. For example, antibiotics or anti-infectives have the ability to kill or inhibit the reproduction of bacteria, so they are prescribed for clients who have an infection. In this example, infection is the indication, or the condition for which the medication is prescribed.

Many medications’ pharmacologic actions have more than one therapeutic use, or indication. For example, diphenhydramine is a medication that clients take for its antihistaminic effect, primarily to treat allergies. You can also give it for motion sickness due to its antiemetic effect, as a hypnotic or sleep aid for insomnia due to its sedative effect, or as part of a treatment plan for Parkinson’s disease due to its anticholinergic effect.

Adverse Drug Reactions (ADR)

ADRs are the nontherapeutic, unintended effects of a medication that occur at a therapeutic dose. They may be predictable and well-known or unpredictable. These can range from annoying but tolerable to life-threatening.

For example, unintended but known effects of diphenhydramine include dry mouth and drowsiness.

Other more serious, possibly life-threatening ADRs include confusion, uncoordination, dizziness, and convulsions in some older adults. It can also cause hyperactivity and excitation in children.

The most severe type of adverse drug reaction is an allergic reaction. The severity of an allergic drug reaction can range from itching and rash or hives to life-threatening anaphylactic shock. Clients who experience a mild drug reaction need to avoid taking the medication again due to the risk of anaphylaxis. The client’s previous exposure increases their sensitivity to the medication and may precipitate a more severe reaction upon second exposure to the medication. 

Anaphylactic shock is an exaggerated response of the body’s immune system to a medication, which precipitates a massive release of histamine and other chemical mediators into the body. Manifestations of anaphylactic shock can occur almost immediately after exposure. Findings include swelling of the eyes, face, mouth, and throat; difficulty breathing; wheezing; rapid heart rate; and extremely low blood pressure. Such manifestations may ultimately lead to cardiac arrest. A client in anaphylactic shock must be immediately brought to a medical facility where they can receive cardiopulmonary support, along with rescue medications.

Treatment of anaphylactic shock focuses on re-establishment of an airway and oxygen therapy. Administer epinephrine to raise the client’s blood pressure and dilate respiratory bronchi and give diphenhydramine to block the additional release of histamine.

Tolerance, Cumulative Effect, and Toxicity

Medication tolerance, cumulative effects, and medication toxicity are other effects you must monitor in your clients during medication therapy.

Medication tolerance is the body’s decreased response to a medication it receives over a period of time. For the medication to continue to exert a therapeutic effect, providers must increase the dosage. Pharmacodynamic tolerance is the term used to describe tolerance associated with long-term use of opioid analgesics. Whereas medication dependence is a physiologic or psychological need for a medication. Withdrawal syndrome can occur in clients who are psychologically and/or physically dependent. The client can experience a variety of physical and mental manifestations when they are no longer taking the medication. A medication interaction is an alteration in the expected pharmacologic action of a medication that results from an interaction with another medication.  

Medication sensitivity, or a cumulative medication effect, may occur in clients secondary to metabolic changes, such as impaired hepatic and renal function, resulting in poor excretion of medications. The body is unable to metabolize and excrete the single dose of the medication before another dose is administered. Nurses should pay particular attention to interventions, such as assessing clinical signs and monitoring laboratory results, that will help reduce ADR in these clients.

Medication toxicity occurs when a client receives medications in excessive dosages, resulting in negative physiologic effects. Providers should prescribe the lowest effective dose possible to achieve therapeutic effects. It can also happen when impaired excretion of the medication allows it to build up in the body. Eventually, this reaches levels where toxic effects are evident. Providers must be cognizant of impaired metabolic and excretory functions of clients and adjust dosages as needed. Periodic laboratory tests are essential in monitoring serum medication levels.

The effects of medication toxicity may be irreversible and life-threatening. For example, vancomycin may cause permanent damage to cranial nerve number eight, resulting in decreased hearing or even deafness. Acetaminophen may cause temporary damage to the liver or permanent damage that results in liver failure.

Precautions and Contraindications

Take precautions when providers prescribe medications that have the potential to cause adverse drug reactions in certain populations or in combination with other medications or certain foods. Only use these medications when necessary and when the benefits outweigh the risks. 

Take extra precautions when providers prescribe medication for clients with chronic or multiple medical conditions. For example, if a client experiences a stroke, that might require anticoagulant therapy. Extra precaution includes physical assessment and laboratory monitoring in order to prevent gastric bleeding if the client also has peptic ulcer disease. 

Other precautions might require limiting certain types of food or concurrent administration of specific types of medications rather than restricting the medication itself. As the next lesson explains, certain combinations of foods and medications can cause an adverse drug reaction. Avoiding these combinations may allow a client to take the medication without consequence.

A boxed warning is the highest safety warning the FDA can place on a group of medications within a class. Providers and clients should be aware of the potential serious side effects that may be caused by a medication with a boxed warning. For example, celecoxib is a non-steroidal anti-inflammatory medication that has a boxed warning related to an increased risk of cardiovascular events and gastrointestinal bleeding.

There are some medications that a provider should not prescribe because they have the potential to cause serious or life-threatening adverse drug reactions. This is called a contraindication. Some medications are contraindicated for use with certain populations or in combination with other medications and certain foods. Providers should not consider the use of medications in these situations except under extremely unusual circumstances. 

Medication-Medication and Medication-Food Interactions

Medication-medication and medication-food interactions can radically change the action of medication in the body. Precautions may require limiting certain types of food or concurrent administration of certain types of medications rather than restricting the medication itself. Grapefruit juice is a drink that can decrease the enzymatic metabolism of certain medications, increasing their potency and risk for toxicity. For example, avoid grapefruit juice if a client is taking the antidepressant sertraline, the antihistamine fexofenadine, or the calcium channel blocker nifedipine. If a client drinks grapefruit juice with these medications, they may accumulate in the body and have serious consequences. Wine and processed meats that contain tyramine can cause a hypertensive crisis in clients taking isocarboxazid or tranylcypromine. The calcium in milk products binds to tetracycline, thereby reducing its absorption. Eating dark green vegetables such as avocados that are high in vitamin K counteracts the anticoagulant effects of warfarin. Finally, ingestion of a meal high in protein decreases the absorption of levodopa, causing manifestations of Parkinson’s to abruptly increase. 

Medication-Food Interaction Examples

Medication-Medication Interactions

Medication- medication interactions occur when one medication changes the way another medication affects the body. When the combined effect of two medications you give together is the same as each medication you give alone in similar doses, an additive effect occurs. This results in a 1 plus 1 equals 2 effect.

For example, when a client takes two medications that are central nervous system depressants, such as alcohol and opioids, their effects add to each other in a 1 plus 1 equals 2 manner, putting the client at risk for significant and possibly fatal central nervous system depression. Synergistic effects occur when the effect of one medication is greater if you give it with another medication. This results in a 1 plus 1 equals more than two effect. For example, when you give aspirin with the prescription blood thinner warfarin, the effects of warfarin intensify and severe bleeding can occur.

Antagonistic effects occur when the effect of one medication is decreased or blocked if you give it with another medication. This results in a 1 plus 1 equals less than two effect. For example, if you give a medication for asthma to open a client's airway and another medication for its cardiovascular effects that has an adverse medication reaction that constricts the client's airways, the effect of the second medication antagonizes or interferes with the action of the first medication, getting a 1 plus 1 equals zero effect.

Medication-Food Interactions

Medication-food interactions occur when a food changes the way a medication affects the body. This can result in increased levels of the medication in the body, which occurs when food increases the amount of a medication the body absorbs or the rate at which it is metabolized. Grapefruit juice, which inhibits an enzyme in the liver that facilitates medication metabolism, can increase the amount of a medication in the body because it no longer breaks down in the liver. This effect occurs if a client takes a medication with grapefruit juice, as well as several hours after drinking grapefruit juice. For this reason, there are several medications that require clients to avoid drinking grapefruit juice during therapy.

Medication-food interactions can also result in decreased levels of the medication in the body, which occurs when food decreases the rate or amount of a medication the body absorbs. The rate at which it is metabolized counteracts the actions of the medication. Clients taking the blood thinner warfarin might see a decrease in the effects of the medication if he or she eats spinach. Spinach contains vitamin K, which enhances the action of certain clotting factors, counteracting the effects of warfarin.

Medication Therapy Across the Lifespan

There are special considerations for certain groups of clients, such as clients who are pregnant or breastfeeding, infants, children, and older adults.

Clients who are pregnant should only take medications that their health care provider prescribes and only after determining that the benefits outweigh the risks. Many medications and foods a client ingests during pregnancy can cross through the placenta and into the fetus through the umbilical cord. Thus, a fetus is at risk for birth defects if a pregnant client takes teratogenic medications during the first trimester. Those medications can cause malformations in a developing embryo or fetus. The FDA previously used a classification system that identified how safe a medication is to take during pregnancy. Medications were placed in categories A, B, C, D, and X. Category D medications carried a possible risk to the fetus, whereas. category X medications demonstrated a relationship between fetal malformations and medication therapy. When providers considered a category X medication for use during pregnancy, they assessed the potential benefits against the risks before making a decision on administration. Implementation of newer FDA pregnancy risk categories and labeling guidelines began in 2015 and is expected to be fully implemented by 2020. The Pregnancy and Lactation Labeling Rule (PLLR) mandates three sections for labeling: pregnancy, lactation, and females and males of reproductive potential. You may access the full report on the FDA website.

Clients who are breastfeeding should only take medications that their health care provider prescribes and only after determining that the benefits outweigh the risks. Pharmacologic effects may occur in infants when medication concentrations in the breast milk are high. Other factors to consider are the size of the infant and the amount of breast milk consumed. 

In neonates, infants, and children, the absorption, distribution, metabolism, and excretion of medications are different from that of adults. For purposes of medication information, clients younger than 1 month of age are neonates (if less than 38 weeks gestation, they are referred to as premature or preterm infants). Clients between the ages of 1 month and 1 year are infants. Clients between the ages of 1 year to 12 years are children. In relation to medication absorption, gastric emptying is slower until 6 to 8 months of age. Gastric pH is higher until 2 years of age. Less of a medication is metabolized in the liver on its first pass-through. If you administer medications intramuscularly in neonates, absorption is delayed and erratic. If you administer medications intramuscularly in infants and children, there is an increase in absorption. The higher water and lower body fat content of neonates, infants, and children affects the distribution of medications. Infants also have a less effective blood-brain barrier than adults, which allows medications to enter the brain more easily. Fewer enzymes are produced by a neonate or infant’s liver until 1 year of age, which affects the metabolism of medications. The rate at which medications metabolize in children from age 1 to 2 years also greatly increases. This rate begins to slow after 2 years of age but does not reach that of an adult until approximately 12 years of age. Excretion of medications significantly decreases until 1 year of age due to decreased perfusion of the kidney, which decreases the rate of filtration and tubular resorption.

Because of the variations in neonates, infants, and children, dosage adjustments in relation to age, weight, and body surface area should be made to avoid toxicity and overdose. 

The physiologic changes that occur with aging also affect the absorption, distribution, metabolism, and excretion of medications and may necessitate an adjustment in the dosage for older adults. Physiologic changes include decreased cardiac output, decreased peristalsis, and increased pH in the gastrointestinal tract, which can affect the absorption and distribution of medications, and decreased renal and hepatic function, affecting medication metabolism and excretion. Kidney and liver function should be reviewed by related laboratory tests before medications are prescribed to older adults. An older adult preparing to undergo surgery for a fractured hip will be prescribed pain medications or analgesics afterward, so liver and kidney function should be reviewed prior to surgery. The amount of anesthetic and recovery time from anesthetic varies significantly related to the client’s ability to metabolize and excrete the anesthetic medications. The dosage of the analgesic and the frequency at which it is administered after surgery need to be based on the ability of the client’s liver and kidneys to metabolize and excrete the medication. 

Client Instructions

Clients need to know about the medications they are taking, so it is your responsibility to instruct the client about these medications as soon as possible following admission to a healthcare facility. This includes the purpose of the medication, the generic and brand name of the medication, the proper method and schedule for administration, potential adverse drug reactions, and precautions the client needs to take during therapy.

Client instruction is critical for the safe administration of medications. When you are preparing to provide client instruction, determine the client’s cognitive and developmental level, learning style, native language, and readiness to learn. Providing instruction to a child or adolescent requires a different approach than what to use with an adult. Integrating play into client instructions is a common method to use with children. Clients who do not speak English need an interpreter to translate the information. This needs to be a medical translator familiar with medical terminology whenever possible, to decrease the chance for misunderstanding. Use a variety of teaching methods to give clients instructions on how to take medication. This can include verbal instructions, written instructions like brochures or pamphlets, videos, or a demonstration of special administration techniques, such as the use of inhalers or giving injections. It’s also a good idea to recommend reputable and accurate Internet resources. Many consumers today are computer literate and rely on the Internet for medical information. Although there are many reputable sources of medication information online, there are many that are not.

After getting instructions, the next step is to evaluate the client’s knowledge by asking specific questions about the medication. If you show them a special administration technique, like an insulin injection, require a return demonstration. Or, ask the client to repeat the information they just heard.

Summary

• A medication has one generic name (the name the manufacturer gives it), and it may have more than one brand name (the name given by a company that is producing and marketing it).

• A provider must prescribe a prescription medication for a client’s use. A nonprescription (over-the-counter) medication may be obtained without a provider’s prescription. 

• Factors that influence the absorption, distribution, metabolism, and excretion of medications include the route of administration, the presence of acute or chronic medical conditions, physiologic changes that occur with aging, and medication-medication or medication-food interactions.

• A medication is prescribed based on its intended therapeutic action (indication).

• An adverse drug reaction is an unintentional, non-therapeutic effect of a medication.

• Precautions may be indicated for medications that may cause adverse drug reactions in certain populations or in combination with some foods or other medications.

• A contraindication for a medication means that it should not be prescribed for a client because it has the potential to cause a serious adverse drug reaction.

• A medication’s half-life is the time it takes for a medication to decrease in amount by half after it enters the body.

• The onset of action is the amount of time it takes for a medication to produce a therapeutic response.

• The peak effect is the time it takes for a medication to produce the full therapeutic effect.

• The duration of action is the length of time the therapeutic effect lasts.

• The peak level of a medication is the point in time at which the medication is at the highest level in the body, and the trough level is the point in time at which it is the lowest.

• Client education is necessary for the safe administration of medications.