system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How can the laxative effects of caffeine or cigarettes onset within minutes, while effects of regular laxatives (e.g. bisacodyl) take hours to manifest?
gpt
Caffeine and Nicotine are stimulant laxatives. They trigger muscle contractions in the intestines to get your bowels moving. They are also delivered through the blood. 

Bisacodyl is only slow acting if you take it orally. It is also a stimulant laxatives, but when taken orally it must be digested before the drug takes effect. If taken as a rectal suppository or enema, the drug acts much faster usually within an hour. Often as fast as 15 minutes.