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Diphenhydramine - Diphenhydramine hydrochloride abbreviated DPH, sometimes DHM .  Trade names Benadryl   Dimedrol, Daedalon. It is also found in the name-brand products Nytol, Unisom , Tylenol [already covered elsewhere], Midol  and Advil , though some Unisom products contain doxylamine instead. It is available as an over-the-counter (OTC) or prescribed HCl injectable.

Diphenhydramine is a histamine H₁receptor antagonist.   Antagonism is achieved through blocking the effect of histamine more than blocking its production or release. Diphenhydramine inhibits most responses of smooth muscle to histamine and acts as a vasoconstrictor by inhibiting the vasodilator effects of histamine. The antagonism may also produce anticholinergic effects, antiemetic effects, and significant sedative side-effects.

It is mainly used to treat allergies. But, like most other first-generation antihistamines, the drug also has a powerful hypnotic effect, and for this reason is often used as a nonprescription sleep aid,  a mild anxiolytic, a motion sickness treatment and even a treatment for the common cold . The drug also acts as an antiemetic. It has been used to provide local anaesthesia.  Diphenhydramine has been shown to build tolerance against its sedation effectiveness very quickly

It has also been used for the treatment of ‘extrapyramidal side-effects’ of many antipsychotics, such as the tremors that haloperidol can cause. In addition, injectable diphenhydramine can be used for life-threatening reactions (anaphylaxis) to allergens such as bee stings, peanuts, or latex, instead of epinephrine.

Side effects due to the potent anticholinergic action include dry mouth and throat, increased heart rate, pupil dilation, urinary retention, constipation, and, at high doses, hallucinations or delirium. Further side-effects include motor impairment (ataxia), flushed skin, blurred vision at nearpoint owing to lack of accommodation, abnormal sensitivity to bright light, difficulty concentrating, short-term memory loss, visual disturbances, irregular breathing, dizziness, irritability, itchy skin, confusion, decreased body temperature (in general, in the hands and/or feet), erectile dysfunction, excitability, and, although it can be used to treat nausea, higher doses may cause vomiting.

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. Considerable overdosage can lead to cardiac arrest, serious ventricular dysrhythmias, coma, and death.

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