Antihistamines: Generic and Brand Names
- First-generation antihistamines
- brompheniramine (Bidhist)
- carbinoxamine (Histex, Palgic)
- chlorpheniramine (Aller-Chlor)
- clemastine (Tavist)
- cyclizine (Marezine)
- cyproheptadine
- dexchlorpheniramine
- dimenhydrinate (Dimetabs)
- diphenhydramine (Benadryl)
- hydroxyzine (Vistaril)
- meclizine (Antivert)
- promethazine (Phenergan)
- triprolidine (Zymine)
- Second-generation antihistamines
- azelastine (Asteline)
- cetirizine (Zyrtect)
- desloratadine (Clarinex)
- fexofenadine (Allegra)
- levocetirizine (Xyzal)
- loratadine (Claritin)
Disease Spotlight: Seasonal Rhinitis
Seasonal rhinitis, commonly called hay fever, is inflammation of the nasal cavity. The upper airways meet a specific antigen (pollen, mold, dust) with a vigorous inflammatory response, producing nasal congestion, sneezing, stuffiness, and watery eyes.
What are Antihistamines?
Antihistamines block the release or action of histamine, a chemical released during inflammation that increases secretions and narrows airways. They appear in many OTC preparations aimed at respiratory symptoms and allergies. Match the drug to the patient: first-generation agents have stronger anticholinergic effects and cause drowsiness, so anyone who needs to stay alert should get a less sedating second-generation agent. Because they are OTC, patients often misuse them for colds and influenza.
Therapeutic Actions
Antihistamines selectively block histamine at the histamine-1 receptor to decrease the allergic response, and carry anticholinergic (atropine-like) and antipruritic effects.
Indication of Antihistamines
Relief of seasonal and perennial allergic rhinitis, allergic conjunctivitis, uncomplicated urticaria, and angioedema; relief of nasal and non-nasal symptoms of allergic rhinitis; relief of dermographism discomfort and adjunctive therapy in anaphylactic reactions; and relief of nausea and vomiting from motion sickness.
Pharmacokinetics of Antihistamines
Antihistamines are well absorbed orally, with an onset of action ranging from 1 to 3 hours.
Contraindications and Cautions
Pregnancy and lactation, unless the benefit to the mother clearly outweighs the risk to the fetus or baby. Renal or hepatic impairment, which can alter metabolism and excretion, so use with caution. Arrhythmias: take special care in any patient with a history of arrhythmias or prolonged QT intervals, because fatal cardiac arrhythmias have occurred with certain antihistamines combined with QT-prolonging drugs, including erythromycin.
Adverse Effects of Antihistamines
CNS: drowsiness and sedation. GI: drying of the GI mucous membranes, GI upset, nausea. GU: dysuria, urinary hesitancy. Skin: eruption and itching.
Interactions
Anticholinergic effects may be prolonged if diphenhydramine is taken with a monoamine oxidase inhibitor (MAOI). Fexofenadine combined with ketoconazole or erythromycin can rise to toxic levels.
Nursing Considerations for Antihistamines
Assess for contraindications and cautions: allergy to antihistamines, pregnancy and lactation, and prolonged QT interval (contraindications), and renal or hepatic impairment (cautious use). Do a physical exam for baseline. Check skin color, texture, and lesions for anticholinergic effects or allergy; orientation, affect, and reflexes for CNS effects; respirations and adventitious sounds for drug effect; and renal and liver function tests for factors affecting metabolism or excretion. Relevant nursing diagnoses include acute pain related to GI, CNS, or skin effects, disturbed sensory perception related to CNS effects, and deficient knowledge regarding therapy.
Give the drug on an empty stomach, 1 hour before or 2 hours after meals, to increase absorption. Response varies between agents, so the prescriber may need to try several to find the most effective one. For the dry mouth these drugs cause (which can lead to nausea and anorexia), suggest sugarless candies or lozenges. Provide safety measures if CNS effects occur. Increase humidity and push fluids to ease thickened secretions and dry nasal mucosa. Have the patient void before each dose if urinary retention is a problem. Provide skin care if dryness and lesions appear. Tell the patient to avoid alcohol, which can cause serious sedation, and to check OTC products for antihistamines to avoid stacking toxic doses. Teach the drug name, dosage, warning signs, and the need for periodic monitoring, and offer support for coping with the disease and regimen. Evaluate by tracking symptom relief, watching for adverse effects (skin dryness, GI upset, sedation, urinary retention, thickened secretions, glaucoma), confirming the patient understands the teaching, and monitoring comfort, safety, and compliance.
Frequently Asked Questions
What is the difference between first-generation and second-generation antihistamines?
First-generation agents (diphenhydramine, hydroxyzine, chlorpheniramine) are lipophilic, cross the blood-brain barrier, and cause sedation and anticholinergic effects. Second-generation agents (loratadine, cetirizine, fexofenadine, levocetirizine) are peripherally selective and minimally sedating, so guidelines recommend them as first-line for allergic rhinitis and chronic urticaria (World Allergy Organization).
Do antihistamines treat colds and the flu?
Not really. Antihistamines target the histamine-driven allergic response (sneezing, itching, watery eyes, hives), not viral infection. Patients often misuse them for colds, and stacking them with multi-symptom OTC products can add up to a toxic anticholinergic dose.
Why do first-generation antihistamines make you drowsy?
They cross the blood-brain barrier and block central H1 receptors, which produces sedation and psychomotor impairment. That is why anyone who needs to drive, operate machinery, or stay alert should choose a second-generation agent, and why alcohol should be avoided since it compounds the sedation.
Are antihistamines safe in pregnancy?
They are used in pregnancy and lactation only when the benefit to the mother clearly outweighs the risk to the fetus or infant, so this is a prescriber decision. Renal or hepatic impairment can also alter metabolism and calls for cautious dosing.
Can antihistamines affect the heart?
Certain antihistamines can prolong the QT interval, and fatal arrhythmias have occurred when they are combined with QT-prolonging drugs such as erythromycin or ketoconazole. Take special care in any patient with a history of arrhythmias or prolonged QT.
Is promethazine safe for young children?
No. The FDA carries a boxed warning against using promethazine in children younger than 2 years because of the risk of severe, sometimes fatal respiratory depression, and it should be used cautiously in older children.