5-HTP can cause a notable decrease in appetite, which is associated with a decrease in body weight.

“So for instance, I didn't know this, but 5-HTP produces a notable decrease in appetite, three studies. And this appetite suppression makes sense, of course, because we ingest foods to get serotonin. And if we have enough serotonin then there's no reason to ingest more foods. It tends to have a blunting of appetite. It probably does that also through other mechanisms. So I'm not saying you should do this but if someone's trying to blunt their appetite could be a interesting route, although I don't recommend chronic use, not surprisingly there's a decrease in body weight as a consequence”