It's not though. Colace is primarily an emollient laxative i.e. it acts by softening stools and is not necessarily as active as the other kinds of laxatives in inducing bowel movements. As such you shouldn't expect it to provide a quick relief and it roughly takes at least half a day to a couple days depending on your diet and internal constitution to see results. It's possibly that delay that made you consider it a placebo perhaps.