Antihistamine: How they work and which one to pick

Got a runny nose, itchy eyes, or hives? Antihistamines are the go-to drugs for those symptoms because they block histamine, the chemical your body releases during an allergic reaction. They don’t fix the allergy trigger, but they stop the itch, sneezing, and watery eyes fast. Here’s a plain-language guide to the main kinds, common side effects, and smart ways to use them.

Types and when to pick them

There are two main groups: first-generation and second-generation antihistamines. First-generation pills like diphenhydramine (Benadryl) and chlorpheniramine work quickly but make you sleepy and can cause dry mouth, blurred vision, and constipation. They’re fine for short-term use—for example, when you need sleep or sudden relief—but avoid them when you need to drive or operate machinery.

Second-generation drugs include cetirizine (Zyrtec), loratadine (Claritin), and fexofenadine (Allegra). These cause much less drowsiness and are better for daily use during allergy season. For eye or nasal symptoms some people find topical options useful: eye drops with olopatadine for itchy eyes, or prescription nasal sprays like azelastine for stubborn nasal symptoms.

Practical safety tips

Follow the label dose. Typical OTC doses: cetirizine 10 mg once daily, loratadine 10 mg once daily, fexofenadine 180 mg once daily (or 60 mg twice). Diphenhydramine is usually 25–50 mg every 4–6 hours, but it will make you drowsy.

Don’t mix antihistamines with alcohol or sedatives — they add up and increase sleepiness. Older adults should avoid first-generation drugs because they raise the risk of confusion, falls, and urinary problems. If you have high blood pressure, heart disease, glaucoma, or prostate enlargement, check with your pharmacist or doctor before starting anything.

Watch for drug interactions. Antihistamines can interact with MAO inhibitors and some antidepressants, increasing side effects. Combination allergy pills that include a decongestant (pseudoephedrine) raise blood pressure and may not be safe if you have hypertension or certain heart conditions.

If symptoms are severe—breathing trouble, swelling of face or throat, or widespread hives—seek emergency care. Also see your doctor if OTC meds stop helping; you may need prescription options, allergy testing, or a different treatment plan.

Questions? Talk to your pharmacist. They can help you pick the right antihistamine for your symptoms, age, and other medications. Small changes—choosing a non-drowsy option, adjusting timing, or switching to a nasal spray—can make daily life much easier when allergies strike.