Introduction to Drug Interactions

A drug interaction may be considered a situation in which the effects of one drug are altered by prior or concurrent administration of another drug (i.e., drug-drug interactions).
The concept of drug interaction often is extended to include situations in which (DFHELD):
1. Food or certain dietary items influence the activity of a drug (i.e., drug-food interactions).
  - For example, grapefruit juice can inhibit the metabolism of certain medications, leading to increased drug levels in the body.
2. Herbs or other natural drug products influence the activity of a drug (i.e., drug-herb interactions).
  - Herbal supplements, such as St. John's Wort, can affect the action of prescribed medications by inducing drug metabolism.
3. Environmental chemicals or smoking (i.e., drug-environment or drug-lifestyle interactions) influences the activity of a drug.
  - For instance, smoking can increase the metabolism of drugs like theophylline.
4. A drug causes alterations of laboratory test results (i.e., drug-laboratory test interactions).
  - Some medications may interfere with lab tests, resulting in misleading results, such as certain antibiotics affecting liver function tests.
5. A drug causes undesired effects in patients with certain disease states (i.e., drug-disease interactions).
  - For example, using beta-blockers in patients with asthma can exacerbate the condition.

Most drugs used in current therapy have the capacity to influence many physiological systems.
Therefore, two drugs concomitantly administered will often affect some of the same systems.
When considering the potential for interactions between drugs, there often is a tendency only to be concerned with the primary effects of the drugs involved and to overlook the secondary activities they possess.
Example: Chlorpromazine + amitriptyline + trihexyphenidyl (CAT) possess anticholinergic activity, leading to additive effects which can cause side effects such as dry mouth and constipation.

It is necessary for some individuals to see more than one physician, and it is common for a patient to be seeing specialists in addition to a family physician.
It is frequently difficult for one prescriber to be aware of all the medications that have been prescribed for a particular patient; thus, potential for drug interactions increases substantially.
Engage in careful communication among prescribers to mitigate this risk.

Patients often neglect to mention nonprescription products they have purchased when questioned by physicians.
Some physicians and pharmacists prefer to use a comprehensive list of symptoms treated by over-the-counter products to gather information about nonprescription product use.
The precautions regarding potential interactions also apply to herbal products and dietary supplements which are widely accessible without prescriptions.

Various reasons contribute to patients not taking medications as intended by the prescriber.
In some cases, patients have not received adequate instruction from the prescriber or pharmacist on how and when to take their medications.
Confusion may arise, especially among patients taking multiple medications, even if initial instructions were understood.
Although noncompliance generally results in underuse of medication, it can sometimes lead to excessive use of some medications, thereby increasing the risk of drug interactions.

The tendencies of some individuals to abuse or misuse drugs deliberately may significantly raise the incidence of drug interactions, as illicit drugs may have unknown interactions with prescribed medications.