Antihistamines and Decongestants

Core Concepts

Histamine and the allergic response

Histamine is a biogenic amine stored in mast cells (in tissues) and basophils (in blood). When an allergen such as pollen binds to IgE antibodies on a sensitized mast cell, the cell degranulates and releases histamine. Histamine then acts on H1 receptors in the nose, eyes, skin, and airways to produce the classic symptoms of allergy: vasodilation (redness, swelling), increased vascular permeability (runny nose, watery eyes), itching, and sneezing. Histamine also acts on H2 receptors in the stomach to stimulate acid secretion — a reminder that "antihistamine" in allergy care almost always means an H1 blocker, a different class from the H2 blockers used for reflux and ulcers.

How antihistamines work

Antihistamines used for allergy are H1 receptor antagonists: they bind to H1 receptors and block histamine from activating them, preventing the symptoms rather than undoing damage already done. That is why they work best when taken before or early in exposure.

The class splits into two generations with a clinically important difference:

• First-generation antihistamines (e.g., diphenhydramine, chlorpheniramine) are small, lipophilic molecules that cross the blood–brain barrier easily. They block H1 receptors in the brain, producing sedation, and they also have anticholinergic effects (dry mouth, dry eyes, urinary retention, constipation). They are effective but can impair driving, school performance, and balance — a particular concern in older adults, where they are linked to confusion and fall risk.
• Second-generation antihistamines (e.g., loratadine, cetirizine, fexofenadine) are designed to stay mostly outside the brain, so they cause far less sedation and fewer anticholinergic effects. They are generally preferred for daytime allergy symptoms, though individual responses vary — cetirizine, for example, still makes some people drowsy.

How decongestants work

Nasal congestion happens when blood vessels in the nasal mucosa dilate and leak fluid, swelling the tissue and narrowing the airway. Decongestants are sympathomimetics that stimulate alpha-1 adrenergic receptors on those vessels, causing vasoconstriction. Shrinking the vessels reduces tissue swelling and opens the nasal passages. The most familiar oral examples are pseudoephedrine and phenylephrine; topical (spray/drop) forms work the same way but are applied directly to the nasal mucosa.

The route changes the trade-offs. Topical decongestants act quickly with little systemic absorption, but prolonged use (beyond the labeling guidance) can cause rebound congestion (rhinitis medicamentosa) — the vessels dilate again as the drug wears off, driving the person to use more. Oral decongestants avoid rebound but are absorbed systemically, so their cardiovascular effects (increased heart rate and blood pressure, jitteriness, insomnia) matter more. In many countries, pseudoephedrine is kept behind the pharmacy counter because it can be diverted to make methamphetamine.

Adverse effects and precautions

• Antihistamines: sedation and anticholinergic effects dominate with first-generation drugs. Caution is warranted in older adults, in people whose jobs require alertness, and with alcohol or other CNS depressants (additive sedation). Some first-generation antihistamines are used as sleep aids — a reminder that "side effect" can become "intended use" in a different product.
• Decongestants: sympathomimetic stimulation can raise heart rate and blood pressure and worsen insomnia, anxiety, or tremor. People with poorly controlled hypertension, significant heart disease, or hyperthyroidism need individual evaluation — always verify current precautions against the formulary and references rather than relying on memorized lists, and note that guidelines and institutional policies vary.

Nursing considerations

• Medication history: ask about all OTC products, including combination cold medicines, because duplicate ingredients (e.g., two antihistamines in different products) are common.
• Assessment: ask about sleepiness, falls, dizziness, dry mouth, urinary symptoms, and cardiovascular symptoms; check blood pressure and heart rate where relevant.
• Teaching: explain the difference between the two classes, when each helps, and the drowsiness warning for first-generation products; advise people not to drive after taking a sedating product and to avoid alcohol. Instruct people using topical decongestants to follow the label's duration guidance to avoid rebound congestion.
• Scope: OTC counseling and medication education fall within nursing practice, but specifics of what nurses may recommend or administer vary by jurisdiction and institution — verify against local policy and prescriber orders.

Common Confusions