It seems that most drugs both illicit and legal cause a dry sensation in the mouth. Why is this?
Perhaps because they’re toxins that your body tries to purge by flushing it out in the urine, which uses up your water, and the body makes you thirsty so you’ll drink something and replenish the supply?
I think you’re being a wee bit teleologic there, Ethilrist.
Some drugs cause a dry mouth due to their inherent pharmacologic properties to dry up saliva (e.g. anti-histamines, tricyclic antidepressants, PCP). Some drugs actually do cause dehydration with dry mouth arising as a result (e.g. Ecstasy). Some others cause a dry mouth as a result of mouth breathing (perhaps due to anxiety or the drugs’ direct effect to stimulate breathing).
I would venture a guess that the drugs in question are stimulating the sympathetic nervous system to do things such as relax bronchial muscles or skeletal muscles (common reasons many of these drugs are prescribed). The same pathway that causes these relaxations, decreases salivary gland secretions.
But this is just my undergraduate level understanding of biology and I could be mistaken.
[nitpick]By and large, the predominant neurotransmitter (or autonomic) cause of these drugs’ ability to dry the mouth is due to their anticholinergic properties, not sympathetic.
Since we’re on the subject, why would some people get cottonmouth from specific drugs while some people don’t? Just asking because everyone else seems to get it from cannabis.
Clearly, you get cottonmouth because the drugs contain snake oil.
A lesser known side effect of snake oil in drugs is copperhead. Kind of nasty, too.
Yes, this is the likely reason.
Lots of drugs have anticholinergic effects, including dry mouth. There are also numerous environmental toxins that do this.
Here’s more than you may want to know about anticholinergic syndrome, including a lovely mnemonic for the signs and symptoms:
“red as a beet, dry as a bone, blind as a bat, mad as a hatter, and hot as a hare.”
Sounds like the average SDMB poster. 
Oh yeah that is part of the mnemonic for the symptoms of people who ingest Datura Stramonium. :eek: