Types of medicine
What Is an Antihistamine? Allergy Medicines Explained
Antihistamine · say it: an-tee-HISS-tuh-meenAlso written: anti-histamine, H1 blocker, allergy medicine, antihistamines
Short answer
An antihistamine is a medicine that blocks histamine, used for allergy symptoms like sneezing, itching and hives. Some cause drowsiness.
What it means
Antihistamines block histamine, a chemical the body releases during allergic reactions. They ease symptoms such as sneezing, runny nose, itchy eyes, hives and itching. Some are also used for travel sickness, nausea or short-term sleep problems. Common examples include cetirizine, loratadine, fexofenadine, chlorphenamine and diphenhydramine.
Older 'first-generation' antihistamines, like diphenhydramine and chlorphenamine, often cause drowsiness, so their labels warn about driving, alcohol and other sedating medicines, and some warn older people about confusion. Newer ones are less likely to cause drowsiness but still can in some people. Labels may also say to ask a doctor first if you have glaucoma, trouble passing urine from an enlarged prostate, or breathing problems.
Allergy antihistamines (H1 blockers) are different from H2 blockers, such as famotidine, which are also technically antihistamines but are used to reduce stomach acid.
Where you may see it
- Over-the-counter allergy and sleep-aid labels
- Cough and cold combination products
- Allergy clinic letters
📄 How it looks on a report
May cause marked drowsiness. Alcohol, sedatives and tranquilizers may increase drowsiness. Be careful when driving a motor vehicle.
🙅 What it does not mean
- 'Non-drowsy' does not mean it can never cause drowsiness.
- It does not treat a severe allergic reaction (anaphylaxis) on its own; that needs emergency help.
- Taking two products with antihistamines together does not work better and can add side effects.
🗒️ Questions to ask your doctor
💊 Medicines whose labels mention “Antihistamine”
Plain-English explanations of each medicine, from its official FDA label.
- Alprazolam
a benzodiazepine, a type of central nervous system medicine
- Cetirizine
an antihistamine of the selective H1-receptor antagonist type
- Diphenhydramine Hydrochloride
an antihistamine (an antihistaminic agent) given by injection into a vein or deep into a muscle
- Fexofenadine
an antihistamine, an allergy medicine sold over the counter
- Loratadine
a non-drowsy antihistamine, a type of medicine that works against allergy symptoms
- Lorazepam
A benzodiazepine, a type of anxiety medicine
🔗 Read more from trusted sources
- “Antihistamine” on MedlinePlus ↗
US National Library of Medicine
- NCI Dictionary definition ↗
US National Cancer Institute: “antihistamine”
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