When pollen meets the lining of your nose, immune cells called mast cells release histamine. That molecule triggers sneezing, itching, watery eyes, and mucus production. Antihistamines block histamine at its receptor, quieting the response. They are among the most-used drugs in the world, and most people have taken one without thinking much about it.

First-generation antihistamines like diphenhydramine cross the blood-brain barrier and cause drowsiness. That side effect made them useful as sleep aids and motion sickness remedies, but it also makes them a poor choice for daytime allergy relief. Second-generation drugs like loratadine and cetirizine were designed to stay out of the brain, so they relieve symptoms without much sedation. Cetirizine still causes drowsiness in some people, a reminder that individual responses vary.

Practical points:

• Allergic rhinitis. Second-generation antihistamines are first-line.
• Hives. Antihistamines reduce itching and wheals.
• Anaphylaxis. Antihistamines are not enough. Epinephrine is the treatment.
• Motion sickness. First-generation drugs like meclizine work well.

Topical antihistamines exist for eye drops and nasal sprays. They act locally and avoid systemic side effects.