Diphenhydramine
BenadrylPubChem CID 3100 · 3D model
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Blocks histamine from triggering the sneezing, itching, and swelling of allergic reactions, and because it slips easily into the brain, it also brings on the drowsiness that's put to use as an over-the-counter sleep aid.
How it works
Diphenhydramine mainly works by blocking histamine — the chemical your body releases during an allergic reaction — which is why it calms a runny nose, itching, and hives. Because it's able to reach the brain, it also has a sedating effect, and it blocks a separate signaling chemical called acetylcholine that plays a role in nausea and in certain movement-related side effects caused by other medications. Its effects kick in within one to a few hours of taking a dose and last around four to six hours. That combination of allergy relief, drowsiness, and anti-nausea action is why it turns up in allergy medicine, sleep aids, and motion-sickness treatments.
What it’s used for
It's approved to relieve allergy and cold symptoms like sneezing, runny nose, itchy/watery eyes, hives, and swelling; as a backup treatment alongside epinephrine for severe allergic reactions; to prevent and treat motion sickness; to ease Parkinson's-related stiffness and tremor when other treatments aren't an option; and as a short-term nighttime sleep aid for occasional sleeplessness.
- Mechanism
- First-generation H1 histamine receptor antagonist
- Drug class
- Small molecule
- Therapeutic area
- Allergy / Immunology