We have a bunch already. There's DNP, which is hard to out-eat, but you need mega-doses of antioxidants to prevent cataracts and shit and ice baths to prevent heat stroke because it's actually an herbicide. Amphetamines are great for weight loss, too, but are schedule II drugs for a reason. There's various gut absorption blocker meds that turn your poo entertaining consistencies and cause things like anal leakage... I expect coming up with a good diet drug is actually a difficult problem.