Diphenhydramine
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| IUPAC chemical name
2-(Diphenylmethoxy) -N,N-dimethylethylamine hydrochloride. | |
| CAS number 147-24-0 | ATC code ? |
| Chemical formula | C17H21NO · HCl |
| Molecular weight | 291.82 g/mol |
| Bioavailability | ? |
| Metabolism | ? |
| Elimination half-life | ? |
| Excretion | ? |
| Pregnancy category | ? |
| Legal status | ? |
| Routes of administration | ? |
| Indicated for: Recreational uses:
Unethical uses:
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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.
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. Despite being one of the oldest antihistamines on the market, it is by and large 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. Consequently, the drug is the most deferred to when an allergic reaction requires a strong and quick reversal of the (often dangerous) effects of a massive histamine release. However, it is not always the drug of choice for treating allergies. Like many other first generation antihistamines, is also 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 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.
Recreational drug users sometimes take several times the recommended dose of diphenhydramine in order to attain an intense hallucinogenic (more accurately, delirious) state. The mental effects are described by many as "dreaming while awake" involving visual and auditory hallucinations which, unlike those experienced with most psychedelics, often cannot be readily distinguished from reality. Many users report a side effect profile consistent with tropane glycoalkaloidal poisoning. This is due to antagonism of muscarinic acetyl choline receptors in both the central and autonomic nervous system, inhibiting various signal transduction pathways. In the CNS, diphenhydramine readily crosses the blood-brain barrier, exerting effects within the visual and auditory cortex, accounting for reported visual and auditory disturbances. 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 extrapyramidal side-effects, and at sympathetic post-ganglionic junctions, causing urinary retention, pupil dialation, tachycardia, and dry skin & mucous membranes. Considerable overdosage can lead to myocardial infarction, serious ventricular dysrhythmias, coma and death. Such a side-effect profile is thought to give ethanolamine-class antihistamines a relatively low abuse liability.
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 and very slightly less sedating.
External links
- Dimenhydrinate FAQ (http://www.erowid.org/pharms/dimenhydrinate/dimenhydrinate_faq.shtml) - Information on abuse as a recreational drug.
- Presciption Information (http://www.pfizer.com/download/uspi_benadryl.pdf) (PDF)