pressure pills. >> so, we're going to crush this. t reporter: okay. one medication stood out-- zantac and its generic version, ranitidine. it had levels of m.d.m.a. higher than considered acceptable. valisure c.e.o. david light: >> the active ingredient in zantac appears to us to be a fundamentally unstable molecule. >> reporter: light reported to the f.d.a. his suspicion that the problem is not just an impurity, but what could happen to the pill inside the body. >> and in a variety of conditions-- conditions of the lab, conditions of the human owdy-- it can break down and then form the carcinogen m.d.m.a. >> reporter: you're saying the ranitidine itself can break down and form m.d.m.a. >> correct. >> reporter: that possibility, that ranitidine could be converted into m.d.m.a. inside the body, was suggested three years ago in research published by william mitch, a professor at stanford university. in a small study, ten volunteers were given a tablet of zantac, and their urine was tested.m.