Anticholinergic drugs are
chemicals that interfere with the action of
Acetylcholine. They normally do this by
blocking the
receptors that
acetylcholine normally activates. That way less of the acetylcholine acts on the receptors before it is broken down.
These drugs have many medical uses, some specifically because of the fact that they are anticholinergic and some for completely other reasons.
Atropine is used to treat some heart conditions, and for pupil dialation. It is also the primary remedy for VX and some other types of Nerve Gas.
Cyclobenzaprine (aka Flexoril) is used as a muscle relaxant, often for back spasms.
Dramamine is used to combat motion sickness, and diphenhydramine is used as an antihistamine (over the counter allergy remedy and used in sleeping pills).
Overdoses of these drugs (or rather large doses, some people do this on purpose, thus its wrong to say its an overdose) have been known to induce vivid hallucinations and last up to 24 hours. Users are often left unable to tell fantasy from reality. One of the most insidious aspects of this is that this portion of the effect normally manifests itself after more than 6 hours, when a naive user would believe that he was coming down and back to normal. (This of course depends on dose, route of administration and what chemical).
Datura, or jimson weed, contains atropine and scopolamine. Often unsuspecting people will smoke it, expecting to get high, and either die or end up in mental institutions.
Belladonna or "Deadly Nightshade" is another well known plant wich contains atropine. It was used in some places (notably Italy) by women to dialate their pupils, because this was said to make them more attractive. It was also said to have been used by witches.