They do? I was under the impression people stopped taking them because they’re massively expensive. And weekly injections is a bit annoying (though should be less annoying than diabetes or death or whatever else).
There is a set of the population, and not a small one, that will quit the drug as soon as they hit the weight they feel is good enough. Even if they can afford it and are used to the shots they just feel that it is done and don't feel like they'll rebound (although most do) once off it.
