First of all it is important to understand that Belviq is not more effective than currently available weight loss drugs like phentermine or Qsymia. But it is different. Because of how it works, it is not a stimulant like phentermine and it is not as sedating as Qsymia. It does however, like all medications, have the potential to produce some side effects and whether these are more tolerable than the side effects of other drugs probably depends more upon the patient than the drug. In other words, Belviq is not a huge breaktrough in weight loss pharmacology. It is a medication that may help some patients lose weight with fewer side effects.
The official labelling for Belviq, worked out between the FDA and its manufacturer warns against prescribing it in combination with stimulant appetite suppressants like phentermine. Although the labelling doesn't explain exactly why this warning was issued, it seems likely that it is designed to prevent some of the problems that occurred with the infamous "fen-phen" diet drug combination that was very popular and later banned in the 1990s.
Belviq has some similarities to fen-phen and no discussion of this medication would be complete without mentioning them. First of all, "fen-phen" was a two-diet-drug combination composed of FENfluramine and PHENtermine and the name derived from the fact that it was simply easier to say "fen-phen" than it was to say "fenfluramine and phentermine". At any rate, fen-phen was very popular because it was very effective. It's extreme effectiveness was a result of the fact that fenfluramine and phentermine each caused weight loss in totally different ways so that there was no cross-tolerance between them. In other words, using fenfluramine and phentermine together generally produced about twice as much weight loss and did either medication alone.
Unfortunately what was unknown at the time was the fact that fenfluramine (but definitely NOT phentermine) caused heart valve damage in some people who took it. Once this became known in 1997, fenfluramine was quickly withdrawn from the US market.
Belviq has some similarity to fenfluramine. Namely, Belviq, like fenfluramine is the kind of medication known as a "serotonin agonist". This means that Belviq, like fenfluramine, works by directly binding to and activating proteins on cells called "serotonin receptors". However, there is a crucial difference between Belviq and fenfluramine. The difference lies with the fact that there are several different types of serotonin receptors and Belviq only binds well to the kind that suppress appetite. Fenfluramine on the other hand bound well to ALL kinds of serotonin receptors and unfortunately it was by binding to one of those types that it caused heart valve damage. Because Belviq does NOT bind to that same receptor, there are good reasons to believe that Belviq is safe.
Indeed, in human trials with Belviq conducted over the last several years, there has been no evidence to suggest that it causes heart valve damage. It is because of both the way Belviq does and does not work as well as the human trial data that the FDA decided to approve Belviq.