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Diphenhydramine hydrochloride (dye fen hye' dra meen), trade name Benadryl as produced by McNeil-PPC a division of Johnson & Johnson, or Dimedrol outside the U.S. & Canada. Unisom and Nytol as sleeping pills) is an over-the-counter (OTC) antihistamine, antiemetic, sedative, and hypnotic. It may also be used for the treatment of extrapyramidal side effects of typical antipsychotics, such as the tremors that haloperidol can cause. It is a member of the ethanolamine class of antihistaminergic agents.

Diphenhydramine is also an anticholinergic and was discovered during the search for synthetic alternatives to scopolamine which would be easier to work with.^{[citation needed]}

Diphenhydramine was one of the first known antihistamines, invented in 1943 by Dr. George Rieveschl, a former professor at the University of Cincinnati.It became the first FDA-approved prescription antihistamine in 1946.

The brand Benadryl is currently trademarked in the United States by Pfizer; however, many drug store chains and retail outlets manufacture less-costly generic versions under their own store brands.

Side effects

Like many other first-generation antihistamines, diphenhydramine is a potent anticholinergic agent. This leads to profound drowsiness as a very common side-effect, along with the possibilities of motor impairment (ataxia), dry mouth and throat, flushed skin, rapid or irregular heartbeat (tachycardia), blurred vision at nearpoint owing to lack of accommodation (cycloplegia), abnormal sensitivity to bright light (photophobia), pupil dilation (mydriasis), urinary retention (ischuria), constipation, difficulty concentrating, short-term memory loss, visual disturbances, hallucinations, irritability, itchy skin, confusion, erectile dysfunction, and delirium. Some side effects such as twitching may be delayed until the drowsiness begins to cease and the person is in more of an awakening mode. Diphenhydramine also has local anesthetic properties, and has been used for patients allergic to common local anesthetics like lidocaine.
Severe, prolonged twitching and muscle spasm have also been experienced.

The most common cardiac dysrhythmias associated with diphenhydramine overdose are sinus bradycardia, elongated S-T segment interval, and premature ventricular contraction.

Diphenhydramine is similar in its effects to dimenhydrinate (Dramamine), its 8-chlorotheophylline salt, although the latter is approximately 60% the potency in terms of required dosage and is slightly less sedating.

Some patients have an allergic reaction to diphenhydramine in the form of hives.

Since 2002 the US FDA requires special labeling: avoid using multiple products including diphenhydramine, or using topical forms to treat poison ivy urushiol-induced contact dermatitis.


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