Diphenhydramine is the primary constituent of dimenhydrinate and dictates the primary effect. The main difference relative to pure diphenhydramine is a lower potency due to being combined with 8-chlorotheophylline. 50 mg of dimenhydrinate contains 27.2 mg of diphenhydramine.
8-Chlorotheophylline was added in order to counteract drowsiness. Theophylline is very closely related to caffeine and theobromine, mild central nervous systemstimulants. It was thought by scientists that by combining the antiemetic effects of diphenhydramine with a stimulant, the extreme drowsiness induced by the former could be mitigated somewhat by the latter. The sedation caused by diphenhydramine, however, is substantially stronger than the stimulation caused by 8-chlorotheophylline, so the overall effect is still mostly sedating. While dimenhydrinate is still used to prevent nausea and emesis, the development of the chemical meclozine has overtaken its usage (marketed as "Dramamine II") because meclozine is less likely to cause drowsiness.
Dimenhydrinate is used as a deliriant at doses of 1200 to 2000 mg, although body weight plays a significant part in dosing of this drug. Slang terms for Dramamine used this way include "dime," "dime tabs," "D-Q," "substance D," "d-house," and "drams." Frequent users of Dramamine are sometimes called Dramatists, a pun on the name. Tripping on Dramamine is sometimes referred to as Dramatizing or "going a dime a dozen," a reference to the amount of Dramamine tabs generally necessary for a trip. The Template:LD50 (the dose at which 50% of animals tested produced fatal symptoms) for dimenhydrinate is 500 mg/kg in lab rats, which may suggest that a human of typical weight would need to ingest a greater than tenfold amount of a psychoactive dose in order to risk death: however, LD50 varies greatly even between mammals and is almost always lower for humans than for rats. As well, it is possible that a significant proportion may experience serious or fatal reactions at doses far lower than the LD50.
Many users report a side effect profile consistent with tropane glycoalkaloidal (e.g. atropine) poisoning as both show antagonism of muscarinic acetylcholine receptors in both the central and autonomic nervous system, which inhibits various signal transduction pathways. In the CNS, dimenhydrinate readily crosses the 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 due to decreased acetylcholine signaling. Toxicology also manifests in the autonomic nervous system, primarily at the neuromuscular junction, resulting in ataxia and extrapyramidal side-effects and the feeling of heaviness in the legs, and at sympathetic post-ganglionic junctions, causing urinary retention, pupil dilation, tachycardia, irregular urination, and dry red skin caused by decreased exocrine gland secretions, and mucous membranes. Considerable overdosage can lead to myocardial infarction (heart attack), serious ventricular dysrhythmias, coma and death. Such a side-effect profile is thought to give ethanolamine-class antihistamines a relatively low abuse liability. The specific antidote for dimenhydrinate poisoning is physostigmine, usually given by IV in a hospital.
Dimenhydrinate has successfully been used as an antiemetic and sedative in housepets. It is commonly used to reduce the effects of idiopathic vestibular syndrome. The suggested dosage is 50 mg for dogs and 10 mg for cats; duration of effect is 8 hours.
This dosage though is not a proper measure for all pets and should be adjusted by weight.