I think there are a few answers. It sort of depends what kind of drugs you're talking about. As far as stimulants go, to include caffeine, ephedrine, ritalin, crystal meth, and cocaine to name a few, I think they do help you play when you're tired or involved in a marathon set (as a disclaimer, I haven't used any of these but caffeine, so I'm not really speaking from experience). They stimulate your nervous system, which makes you more awake, alert, sharp, and focused, which are good for pool shooting. Over the long term, though, you build up tolerance, and when not under the influence you become slower and less sharp, and eventually you can find yourself using the drugs just to get back to a normal level of nervous system function. So there are some definite down-sides.
If you're talking about some of the more recreational drugs, like pot and alcohol, I think they only give people the illusion that they shoot better high. Sober people around them don't often share this illusion. On the other hand, if you're addicted to any drug, or so used to operating under the influence that being sober is either uncomfortable or painful, I bet you do play worse when you're sober and better when you're high. But it's not being high that makes you better, it's the fact that you can't function without the drug that makes you worse.
That sounded kinda preachy, looking back on it. Oh well, drugs are often bad, kids. Exercise moderation.
-Andrew