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Structure formula of Diphenhydramine hydrochloride

Diphenhydramine hydrochloride (trade name Benadryl®, or Dimedrol outside the US) is an over-the-counter (OTC) antihistamine and sedative. It is also given in conjunction with typical antipsychotics to prevent tardive dyskinesia. It is a member of the ethanolamine class of antihistaminergic agents, and its chemical formula is C17H21NO · HCl.

Unlike true antihistamines, which prevent the release of histamine, diphenhydramine works by blocking the effect of histamine at H1 receptor sites. This results in effects such as the reduction of smooth muscle contraction, making diphenhydramine a popular choice for treatment of the symptoms of allergic rhinitis, hives, motion sickness, and insect bites and stings.

Diphenhydramine is a first generation antihistamine drug, and like many other members of that group, is also a potent anticholinergic agent. This leads to drowsiness as a very common side-effect, with other side-effects possible, including sedation, motor impairment, dry mouth and throat, flushed skin, rapid or irregular heartbeat (tachycardia), blurred vision at nearpoint due to lack of accommodation (cycloplegia), abnormal sensitivity to bright light (photophobia), urinary retention, constipation, difficulty concentrating, short-term memory loss, visual disturbances, hallucinations, confusion, and delirium.

The drug is sometimes used recreationally for its hallucinogenic properties in high doses, usually by teenagers without access to alcohol or other drugs seeking a "buzz". This can be very dangerous, as the large doses required to attain the desired effects can easily cause strong adverse reactions, requiring medical attention. In addition, the hallucinations are quite different in nature to those induced by commonly used substances, such as LSD or magic mushrooms, being similar to the effects of tropane alkaloids, where the user is often unaware that he/she is hallucinating and may endanger his/her or others' lives by actions in response to a hallucination, such as a perceived threat.

The hallucinations also rarely have a positive quantitation, and can be frightening and/or unpleasant. The most common reported experiences are a series of extensive contact and conversations with entities that aren't there, including supernatural beings, friends, relatives or even dead acquaintances. The user may occasionally converse for over 10 minutes with an imaginary being, without ever realising the irrationality of the situation, although often attempts at physical contact with such hallucinations reveal their true nature, and are reported to simply disappear when touched.

Diphenhydramine is very similar in its effects to dimenhydrinate (Dramamine®), its 8-chlorotheophyllinate salt, although the latter is approximately 50% less potent in terms of required dosage.

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