Antihistamines: How They Work and What You Should Know

Antihistamines are the go-to meds for runny nose, itchy eyes, hives, and some forms of nausea. They block histamine, a chemical your body releases during an allergic reaction. That simple action can stop sneezing, itching, and swelling fast. But not all antihistamines act the same, and knowing the differences makes a big difference in how you feel and how safe the drug is for you.

First vs. second generation: pick the right one

First-generation antihistamines (like diphenhydramine) cross into the brain and cause drowsiness. They work quickly, but you may feel sleepy for hours. Second-generation drugs (cetirizine, loratadine, fexofenadine) cause much less drowsiness and are better for daily use. For kids, the choice depends on age and the symptom: short-term intense itching or sleep issues might need different choices than long-term allergy control.

If you’re shopping or talking to a pharmacist, ask whether a product is sedating. That matters if you drive, work with machines, or mix alcohol or other sedatives. Older adults and people with prostate problems or glaucoma should be cautious with sedating antihistamines because those drugs can make certain conditions worse.

Practical dosing and safety tips

Always follow the label or your doctor’s directions. Many antihistamines have over-the-counter options with clear dosing; others need prescriptions. For infants and small children, dosing usually depends on weight, not age—so weigh the child or check the weight chart. Our guide "Hydroxyzine Dosing for Infants" on this site explains weight-based examples and real safety tips if your child needs hydroxyzine.

Watch for interactions. Antihistamines that cause drowsiness can add to the effects of opioid painkillers, sleep aids, or alcohol. Combining those increases fall risk and breathing problems. Also, some antihistamines can interact with heart medicines or strong antifungals—tell your prescriber what else you take.

Side effects to expect: dry mouth, dry eyes, constipation, and blurred vision with older antihistamines. If you notice fast heartbeat, severe dizziness, or trouble breathing, stop the drug and get medical help. For long-term allergy control, second-generation antihistamines are usually safer and better tolerated.

Want non-drug options? Try nasal saline rinses for congestion, allergen-proof bedding, and HEPA filters in the home. For skin hives, cool compresses and short-term antihistamine use often help. If allergies persist or affect sleep, talk to a doctor about allergy shots or prescription options.

For more detailed reads on meds that relate to allergy care—like hydroxyzine dosing, sedating drugs, or how certain prescriptions compare—check the linked articles in our antihistamines tag. If you have questions about a specific medication, bring the drug name and your other meds to your appointment so your clinician can give targeted advice.