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What is/are Diphenhydramine?

Diphenhydramine (/ˌdaɪfɛnˈhaɪdrəmiːn/; abbreviated DPH, sometimes DHM) is a first-generation antihistamine possessing anticholinergic, antitussive, antiemetic, and sedative properties that is mainly used to treat allergies. It is also used in the management of drug-induced parkinsonism and other extrapyramidal symptoms. The drug has a strong hypnotic effect and is FDA-approved as a non-prescription sleep aid, especially in the form of diphenhydramine citrate. It is produced and marketed under the trade name Benadryl by McNeil-PPC (a division of Johnson & Johnson) in the U.S., Canada and South Africa (trade names in other countries include Dimedrol, Daedalon and Nytol). It is also available as a generic or store brand medication.

Medical uses

Diphenhydramine is a first-generation antihistamine used to treat a number of conditions including allergic symptoms and itchiness, the common cold, insomnia, motion sickness and extrapyramidal symptoms.

Diphenhydramine is significantly more potent in treatment of allergies than a newer generation of antihistamines. Consequently, it is frequently used when an allergic reaction requires fast, effective reversal of a massive histamine release. Diphenhydramine is available as an over-the-counter drug (OTC) or prescription-only solution for injection. Injectable diphenhydramine can be used for life-threatening reactions (anaphylaxis) to allergens such as bee stings, peanuts, or latex, as an adjunct to epinephrine.

As a potent antagonist to acetylcholine in muscarinic receptors, diphenhydramine is used to treat Parkinson's disease-like extrapyramidal symptoms caused by typical antipsychotics. The muscarinic receptor antagonism leads to correction of levels of dopamine, the neurotransmitter responsible for control of motor function in the brain, similar to other antimuscarinic agents such as atropine. Diphenhydramine can cause strong sedation and has also been used as an anxiolytic as a result.

Because of these sedative properties, diphenhydramine is widely used in nonprescription sleep aids for insomnia. The maximum recommended dose is 50 mg (as the hydrochloride salt), as mandated by the U.S. FDA. The drug is an ingredient in several products sold as sleep aids, either alone or in combination with other ingredients such as acetaminophen (paracetamol). An example of the latter is Tylenol PM. Examples of products having diphenhydramine as the only active ingredient include Unisom, Tylenol Simply Sleep, Nytol, ZzzQuil, and Sominex (the version sold in the US and Canada; that sold in the UK uses promethazine). Tolerance against the sedating effect of diphenhydramine builds very quickly; after three days of use at the common dosage, it is no more effective than a placebo.

Diphenhydramine also has antiemetic properties, which make it useful in treating the nausea that occurs in motion sickness. As it causes marked sedation in many individuals, newer-generation antihistamines, including loratadine, cetirizine, and dimenhydrinate, may be preferred for antiemetic use.

There are also topical formulations of diphenhydramine available, including creams, lotions, gels, and sprays. These are used to relieve itching, and have the advantage of causing much less systemic effect (i.e. drowsiness) than oral forms. Diphenhydramine also has local anesthetic properties, and has been used as such in patients allergic to common local anesthetics like lidocaine.

Adverse effects

Diphenhydramine is a potent anticholinergic agent. This activity is responsible for the side effects of dry mouth and throat, increased heart rate, pupil dilation, urinary retention, constipation, and, at high doses, hallucinations or delirium. Other side-effects include motor impairment (ataxia), flushed skin, blurred vision at nearpoint owing to lack of accommodation (cycloplegia), abnormal sensitivity to bright light (photophobia), sedation, 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.
Torsades de pointes can occur as a side effect of diphenhydramine

Acute poisoning can be fatal, leading to cardiovascular collapse and death in 2–18 hours, and is generally treated using a symptomatic and supportive approach.] Diagnosis of toxicity is based on history and clinical presentation, and specific levels are generally not useful. There are several levels of evidence strongly indicating diphenhydramine (similar to chlorpheniramine) can block the delayed rectifier potassium channel and consequently prolong the QT interval, leading to cardiac arrhythmias such as torsades de pointes.

There is no specific antidote for diphenhydramine toxicity, but the anticholinergic syndrome has been treated with physostigmine for severe delirium or tachycardia.

Some patients have an allergic reaction to diphenhydramine in the form of hives. Paradoxically, restlessness or akathisia can also be a side effect that is made worse by increased levels of diphenhydramine. As diphenhydramine is extensively metabolized by the liver, caution should be exercised when giving the drug to individuals with hepatic impairment.

Measurement in body fluids

Diphenhydramine can be quantitated in blood, plasma, or serum. Gas chromatography with mass spectrometry (GC-MS) can be used with electron ionization on full scan mode as a screening test. GC-MS or GC-NDP can be used for quantification. Rapid urine drug screens using immunoassays based on the principle of competitive binding may show false-positive methadone results for patients who have ingested diphenhydramine. Quantitation can be used monitor therapy, confirm a diagnosis of poisoning in hospitalized patients, provide evidence in an impaired driving arrest, or assist in a death investigation.

Mechanism of action

Diphenhydramine is an inverse agonist of the histamine H1 receptor. It is a member of the ethanolamine class of antihistaminergic agents. By reversing the effects of histamine on the capillaries, it can reduce the intensity of allergic symptoms. Diphenhydramine also crosses the blood–brain barrier (BBB) and antagonizes the H1 receptors centrally. Its effects on central H1 receptors cause drowsiness.

Like many other first-generation antihistamines, diphenhydramine is also a potent antimuscarinic (a competitive antagonist of muscarinic acetylcholine receptors), and, as such, at high doses can cause anticholinergic syndrome. The utility of diphenhydramine as an antiparkinson agent is the result of its blocking properties on the muscarinic acetylcholine receptors in the brain.

Diphenhydramine also acts as an intracellular sodium channel blocker, which is responsible for its actions as a local anesthetic. Diphenhydramine has also been shown to inhibit the reuptake of serotonin. Finally, diphenhydramine has been shown to be a potentiator of analgesia induced by morphine in rats.

This article uses material from the Wikipedia article Diphenhydramine, which is released under the Creative Commons Attribution-Share-Alike License 3.0.

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