Pharmacology of Antihistamines, Leukotriene Inhibitors, and Decongestants

Histamine Receptors and Systemic Responses\n\n* H2 Receptor Blockade: Blocking H2 receptors serves to decrease stomach acid production and inhibit the release of acetylcholine. \n* Acetylcholine and Cholinergic Responses: Acetylcholine is responsible for cholinergic responses, which commonly affect the GI system and the bronchioles (e.g., causing bronchoconstriction or heavy secretions).\n* Mechanism of Action: Antihistamine H1 receptor blockers (antagonists) are primarily used to treat various allergic reactions, ranging from nasal stuffiness to reactions resulting from blood clots.\n\n# First-Generation Antihistamines\n\n* Characteristics and Availability: First-generation drugs are typically available over the counter (OTC). They are known for crossing the blood-brain barrier, leading to significant central nervous system (CNS) effects.\n* Key Medications: \n * Diphenhydramine (Benadryl): A widely used first-generation antihistamine.\n * Dimetap (or dimer tap): Previously used for children; known for causing significant drowsiness and having a sedative effect.\n* Side Effects and Patient Education: \n * Drowsiness: Patients must be warned not to drive or operate heavy equipment.\n * Anticholinergic Response: This includes being \"dried out,\" urinary retention, twitchiness, restlessness, and hyperexcitability. This is particularly common in the very young and very old (known as a paradoxical effect).\n * Urinary Retention: Nurses must check the patient’s output and monitor for an enlarged prostate, as these drugs can interfere with bladder emptying. A bladder scan may be required.\n * Sleep Aid Caution: Many OTC sleep aids, such as Tylenol PM, contain diphenhydramine. Patients (especially the elderly) must be taught to read the back of boxes to avoid accidental overdose by taking multiple products containing the same ingredient.\n * CNS Depressants: First-generation antihistamines will worsen the effects of other CNS depressants or seizure medications (e.g., Keppra).\n * Agitation and Confusion: Repeated dosing (e.g., by the third dose) in elderly patients or children may lead to severe agitation, jumpiness, and loss of orientation (not knowing name, age, or location).\n\n# Second-Generation Antihistamines\n\n* General Features: These drugs generally have fewer CNS side effects compared to the first generation, though some sedation can still occur.\n* Key Medications: \n * Fexifenadine (Allegra): Specifically Allegra D or Allegra Hives is often used for hives (urticaria).\n * Loratadine (Claritin): Can be taken during the daytime. \n * Cetirizine (Zyrtec): Can cause drowsiness and is recommended to be taken in the evening.\n * Levocetirizine (Xyzal): A derivative of cetirizine that also has a sedating effect.\n* Administration and Side Effects: \n * Urinary Retention: Like first-generation drugs, these can cause urinary retention, especially in men with prostate issues.\n * Timing of Dosing: Zyrtec and Xyzal should be scheduled for the evening or nighttime to manage daytime sleepiness.\n * Grapefruit Juice Interaction: Patients should avoid grapefruit juice when taking fexifenadine (Allegra). A component in the juice (specifically related to the metabolism involving the GP-system) decreases the drug's effectiveness and longevity by altering how it is metabolized.\n\n# Leukotriene Inhibitors and Mast Cell Stabilizers\n\n* Leukotriene Inhibitors: These drugs modify the leukotriene response and are frequently used for asthma patients and those with severe allergic responses in the scalp.\n * Montelukast (Singulair): The prototype for this class. \n * Zyflo (Zileuton): An alternative to Singulair, though it requires more frequent dosing ($1$ or $2$ hours before a meal) as it is best absorbed on an empty stomach.\n * Safety Monitoring: Patients must report yellowing of the eyes/skin (jaundice) or darkening of urine, which indicates hepatotoxicity. Families should also watch for changes in behavior or mood.\n * Clinical Note: These are maintenance medications and are NOT for the treatment of acute asthma attacks.\n* Mast Cell Stabilizers: These prevent the mast cell membrane from opening and releasing histamine.\n * Cromolyn Sodium: Administered via nasal spray or inhalation.\n * Side Effects: Local irritation, stinging, and burning in the nasal lining. Symptoms of irritation are side effects, not signs of infection.\n * Nursing Care: Patients should rinse and gargle after using a nebulizer and must use the drug daily as prescribed for maximum symptom reduction.\n\n# Decongestants\n\n* Mechanism: Decongestants reduce swelling in the nasal passages. They are divided into two types: sympathomimetics and corticosteroids.\n* Sympathomimetics: These produce or minimize stimulation of the sympathetic (adrenergic) nervous system to decrease inflammation.\n * Oxymetazoline (Afrin): A nasal spray that provides immediate relief.\n * Phenylephrine (Neo-Synephrine): Another common sympathomimetic.\n * Pseudoephedrine (Sudafed): An oral option, often restricted/locked up because it is used to manufacture methamphetamine.\n * Caution: Afrin should only be used every $12\,hours$ for a maximum of $3\,days$. Continued use beyond this leads to \"rebound nasal congestion.\"\n * Systemic Risks: These can increase blood pressure, cause urinary retention, and increase intraocular pressure. They must be used with caution in patients with high blood pressure, heart disease, glaucoma, or prostate enlargement. Sudafed can also cause heart rate changes like SVT.\n* Nasal Corticosteroids: These decrease inflammation in the swollen nasal membranes and prevent mast cells and white blood cells from releasing histamine.\n * Medications: Fluticasone (Flonase) and Triamcinolone (Nasocort).\n * Patient Education: Relief is not immediate but is more appropriate for long-term use than sympathomimetics. Patients must avoid swallowing the spray to prevent nausea and stomach upset. \n * Infection Risk: Because they decrease the immune response, patients are at risk for fungal infections (yeast). Patients must monitor for white patches in the nose or throat.\n\n# Questions & Discussion\n\n* Q: Can Benadryl be taken during pregnancy?\n * A: It is often prescribed for sleep or allergies under provider direction, but it should be used cautiously and not close to delivery. Benadryl cream may be a safer alternative for localized hives as it does not pass through the placenta as readily as systemic forms.\n* Q: Is Hydroxyzine (Atarax/Vistaril) used for allergies?\n * A: Yes, it is an anti-itch (antipruritic) medication often used for general itchiness, hives, or skin reactions from parasites like lice or scabies. It is also used as an anti-anxiety medication and is frequently preferred over Benadryl for elderly patients because Benadryl is too sedating.\n* Q: Why do antihistamines help with migraines?\n * A: Migraines involve an inflammatory response and vasodilation. By stopping the histamine/inflammatory response, these medications (including H2 blockers like Pepcid) can sometimes mitigate migraine symptoms.\n* Q: What is the risk of using Benadryl for sleep long-term?\n * A: Studies suggest it is not good for long-term use due to the risk of anticholinergic response, particularly in the young and the elderly.\n* Anecdote regarding Laundromats and Social Interactions: The speaker and students shared various personal stories: \n * A student's son had a confrontation at a laundromat with a man (referred to as a \"Kevin\") who was impatient about using a washer.\n * The speaker described an incident at a laundromat where individuals (referred to as \"dike bitches\") attempted to remove her laundry from a dryer before it was finished to use the machine themselves. The speaker confronted them, asserting she was willing to use self-defense if her belongings were touched again.\n * A discussion occurred regarding various slang terms for social archetypes (e.g., \"Karen,\" \"Megan,\") and subculture identifiers like \"stud\" and \"stem.\"\n * The speaker mentioned a student who received their license at age $16$ versus others who waited until $18$ or $20$.", "title": "Pharmacology of Antihistamines, Leukotriene Inhibitors, and Decongestants" }```

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