I used to have a friend (she's passed away 2 years ago) who would take a laxative (something that will make you shit) at night, so in the morning she'd be able to completely clear her bowels. And she swore that hit made her tummy flat.
Yes, her tummy was flat (but so was mine, and I never took laxatives). And No, she did not die from it (it was some other tragedy, too personal to tell you about, and it's not my story to tell).
I don't think it's wise to take it every night. But I really don't see the harm in taking it perhaps on the night before a bikini photoshoot or something, or a beach party, or whenever you desperately want your tummy to be at its flattest.