Dimenhydrinate

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

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Recreational use

Dimenhydrinate is mainly used recreationally because of its hallucinogenic effects. A person wishing to use dimenhydrinate recreationally usually takes several times the recommended dosage. This results in an intense, long-lasting state of anti-cholinergic delirium with physiological and psychological effects, including hallucinations. The dosage to experience these effects varies, but is generally upwards of 300mg.

Users report that in fifteen minutes to an hour, the drug begins to have an effect, which includes hallucinations and a confusing "high". The psychological effects are often described as "dreaming while awake", involving visual and auditory hallucinations that are distinctly different from those experienced with other psychedelic drugs. These hallucinations most often cannot be readily distinguished from reality.

The physical effects people commonly experience vary, but usually include the sense of their body being unusually 'heavy,' especially in the legs, to the point where they cannot even walk, or where they constantly bump into things due to lack of motor control. Users commonly think that they are in a completely different environment than they are actually in. Cognition is extremely slowed down by dimenhydrinate, making interaction with other people very confusing and frustrating. Hearing is affected, mainly because users have to "concentrate" to hear sounds.

Users report many extremely unpleasant side effects, a profile consistent with tropane glycoalkaloidal poisoning, caused by the excessive dose of diphenhydramine, as well as theophylline. Because dimenhydrinate is a combination of these two drugs, the side effects are virtually identical to the combined side effects of the consituent drugs. These side effects include dry mouth and eyes, rapid heart beat (tachycardia), somnolence, and extreme malaise/dysphoria. In some cases insomnia is reported, although more often extreme drowsiness (somnolence) occurs. Muscle spasms, twitches and extreme anxiety and amnesia are also very common side effects. This is due to antagonism of muscarinic acetylcholine 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.

Toxicity also manifests itself 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 dilation, tachycardia, and dry skin/mucous membranes. Considerable overdosage can lead to myocardial infarction, serious cardiac arrhythmias, coma and death. Such a side effect profile is thought to give the ethanolamine-class antihistamines a relatively low abuse potential.

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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