If yo u want something that is safe and natural use prune juice, it takes about 2 hours to work,
I have used theis many times in the past, what you do is drink 2 large glasses, if you can use it at room temprature or it can be cold, but it does work a little bit faster at room temp. then have a sandwich or something to help push it along.
After about 2 hours it will start working, I didn't like the time involved with any laxitives your always waiting for them to work and afterwards for them to stop, I only do enemas now much quicker and you are done in mins. instead of hours.