Not to give inept dangerous medical advice or anything, and water seems like the sanest and safest approach, but also, in escalating degrees of severity:
* Adding coffee if that tends to make you pee, or something else that tends to make you pee.
* Doing a salt water flush where you drink a bunch of salted water (not sea water) at body temperature, as described in the Master Cleanse Diet, which if lucky will tend to cause you to flush your bowels quickly.
* If the drug in question has some known antidote or counteracting thing you can take.
* Induce vomiting?
* Stomach pumps!
* Blood transfusion!