Diphenhydramine

Pharmacological action

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.

In the 1960s it was found that diphenhydramine inhibits chlorpheniramine, also an antihistamine.

Common use and dosage

Diphenhydramine is a first-generation antihistamine drug. Despite being one of the oldest antihistamines on the market, it is largely the most effective antihistamine available either by prescription or over-the-counter, and has been shown to exceed the effectiveness of even the latest prescription drugs.^[3] Consequently, it is frequently used when an allergic reaction requires fast, effective reversal of the often dangerous effects of a massive histamine release. However, it is not always the drug of choice for treating allergies.

It is known that diphenhydramine has sedative properties. Many new antihistamines have been introduced without the side effect of sedation. The drug is also used as a sleep aid and is an ingredient in many sleep aids, such as Doxylamine, a different active ingredient^[4]), and most notably Tylenol PM where it is combined with acetaminophen (paracetamol), and Nytol and Sominex which have diphenhydramine as the only active ingredient. Several generic and store brands of antihistamines and sleep aids also contain solely diphenhydramine.

Diphenhydramine is widely used in Unisom.^{[citation needed]}

There are also topical formulations of diphenhydramine available, including creams, lotions, gels, and sprays. They are used to relieve itching, and have the advantage of causing much less systemic effect (i.e. drowsiness) than oral forms.^{[citation needed]}

Side effects

Like many other first-generation antihistamines, diphenhydramine is a potent lidocaine.^[5] 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.^{[citation needed]}

Recreational use

Those who use diphenhydramine recreationally take a higher dose than recommended (usually between 150mg and 850mg)^[6] for its glycoalkaloidal poisoning. This is due to blood-brain barrier, exerting effects within the visual and auditory cortex.

Other CNS effects occur within the limbic system and hippocampus, causing confusion and temporary amnesia. Toxicology also manifests in the autonomic nervous system, primarily at the neuromuscular junction, resulting in ataxia and physostigmine, usually given by IV in hospital.

See also

• Akathisia
• Acrivastine (UK Benadryl Benadryl Allergy Relief)
• Cetirizine (UK Benadryl Once-A-Day)

References

Notes

1. ^ http://www.nytimes.com/2007/09/29/business/29rieveschl.html?ref=health
2. ^ James Ritchie. UC prof, Benadryl inventor dies. Business Courier of Cincinnati. Retrieved on 2007-10-23.
3. ^ Raphael GD, Angello JT, Wu MM, Druce HM (2006). "Efficacy of diphenhydramine vs desloratadine and placebo in patients with moderate-to-severe seasonal allergic rhinitis.". Ann Allergy Asthma Immunol 96 (4): 606-614. PMID 16680933.
4. ^ Chattem: Unisom.
5. ^ http://www.postgradmed.com/issues/1999/08_99/smith.htm
6. ^ http://www.erowid.org/pharms/diphenhydramine/diphenhydramine_dose.shtml

Sources

• Bruce G. Charlton, Self-management of psychiatric symptoms using over-the-counter (OTC) psychopharmacology: the S-DTM therapeutic model - self-diagnosis, self-treatment, self-monitoring. Medical Hypotheses 2005; 65: 823-828.
• J.A. Lieberman, History of the use of antidepressants in primary care. Primary Care Companion, Journal of Clinical Psychiatry 2003; 5 (supplement 7).