dimenhydrinate n : antihistamine and antiemetic (trade name Dramamine) used to treat motion sickness [syn: Dramamine]

Dimenhydrinate (marketed under brand names Dramamine, Gravol and Vertirosan) is an over-the-counter drug used to prevent nausea and motion sickness.

It is most commonly used as pills, although it is also available in liquid form and in suppositories. This last is particularly useful in the case of persistent vomiting.

It is closely related to diphenhydramine HCl, or Benadryl. The differences relate to the weight-for-weight potency (50 mg dimenhydrinate contains 29 mg of the drug diphenhydramine), delay of action (dimenhydrinate must dissociate into diphenhydramine and its counterion in the body before it is active, therefore diphenhydramine produces effects sooner), and degree of sedation produced. Chemically, dimenhydrinate is a salt of two drugs: diphenhydramine and 8-chlorotheophylline, a chlorinated derivative of the theophylline. Theophylline is very closely related to caffeine and theobromine, mild central nervous system stimulants. It was thought 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. In actuality, the sedation caused by diphenhydramine is substantially stronger than the stimulation caused by chlorotheophyllinate. Diphenhydramine, an ethanolamine-class antihistamine, is found in most OTC sleep aids and allergy preparations, such as Tylenol PM and Benadryl. It is primarily a H1-antagonist, but also possesses an antimuscarinic effect. It is used in Dramamine to prevent nausea and emesis; however, the development of the chemical meclizine has overtaken its usage (marketed as "Dramamine II") because meclizine doesn't produce as much drowsiness.

Recreational use

Dimenhydrinate is sometimes used recreationally as a deliriant at doses of 300 - 1400 mg, the LD-50 of dimenhydrinate is 500 mg/kg.

The mental effects can be described as "dreaming while awake" involving visual and auditory hallucinations that, unlike those experienced with most psychedelic drugs, often cannot be readily distinguished from reality. People who consume a high recreational dose can possibly find themselves interacting normally with a hallucination and have normal conversations with for example a hallucination of a family member. A common sometimes frightening hallucination is of insects and spiders. Hallucinations are more frequent and unpleasent in a setting with little or no light. Inexperienced users of deliriants are liable to panic.

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 acetycholine. 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 hospital.

The intoxication can be described along with other deliriants with the following saying “blind as a bat, dry as a bone, red as a beet, mad as a hatter, and hot as a hare."

Veterinary use

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 (2-4 mg per pound) and 10 mg for cats; duration of effect is 8 hours.

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