Where on the web did you see that it was good for weight loss? Was it on sites that were advertising it? That's the only place I can find similar claims. Supplement manufacturers (as well as health food store workers, websites, etc) can legally make any claims they want to make in an effort to sell a product, and do not have to prove it's true unless they claim to cure a disease such as cancer. They often quote 'studies' that either don't exist or are biased and not published in medical journals. Keeping that in mind, the supplement makers only 'claim' to weight loss with this product is to say that it might reduce appetite. If appetite control is a problem, we'd suggest asking your doctor for a prescription that is proven to help with appetite, and for which side effects are known and you can be monitored.
Berkeley Wellness (University of California) says :
Just how safe is it?
Reported side effects include nausea, vomiting, and difficulty breathing. High doses of 5-HTP can cause agitation, fast heart rate, a boost in blood pressure—and in rare cases, coma and even death. Combining it with an antidepressant, any other drug that affects serotonin levels (such naratriptan or sumatriptan, used to treat headaches), or “herbal antidepressants” such as St. John’s wort can also cause such side effects. People who have heart disease, peptic ulcers, kidney disease, or clotting disorders should definitely not take this supplement.
The potential dangers of 5-HTP outweigh any possible benefits.