diphenhydramine

noun

1. (countable, uncountable)An antihistamine and anticholinergic drug that blocks the effect of histamine at H₁ receptor sites, relieving allergy symptoms. It also reduces smooth muscle contraction, increases heart rate, and sedates the user by blocking muscarinic acetylcholine receptors (mAChRs).

   “Diphenhydramine is a convenient and cheap antihistamine for use, but there are other drugs that do not make the user tired.”