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") due to the fact that meclizine doesn't produce as much drowsiness.

Recreational use
A person wishing to seek recreational drug use from dimenhydrinate takes several times the recommend dosage to achieve an intense, long-lasting state of anti-cholinergic delirium which results in physical and mental effects producing hallucinations. The dosage to experience these effects varies but is generally from 300mg to 800mg.

Users report in less than fifteen minutes and sometimes upward of an hour, a hallucinogenic effect and a confusing high. The mental effects are often described as "dreaming while awake" involving visual and auditory hallucinations unlike those experienced with recreational drugs known as psychedelics, that 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, so much as to the point where they can't even walk, or where they constantly bump into things due to lack of control of their limbs. People commonly think that they are in a completely different environment without realizing it, which makes the drug especially dangerous. Reaction times are extremely slowed down with dimenhydrinate, making interaction very confusing and frustrating with other people. In some cases, thoughts pass in under a minute, which makes focusing very difficult if not impossible. Hearing is affected, mainly due to the fact that users have to "concentrate" to hear sounds.

Users report unpleasant side effects, a profile consistent with tropane glycoalkaloidal poisoning. This includes dry mouth and eyes, rapid heart beat (tachycardia), somnolence, and extreme malaise. Though in some cases insomnia is reported, often extreme drowsiness occurs. Spasms, twitches and extreme anxiety and amnesia can plague users, often causes extreme frustration. Amnesia subsides as the physical effects wear off. 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. 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 dilation, tachycardia, and dry skin & mucous membranes. Considerable over dosage 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.

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.