At a really fundamental level, yes, it boils down to calories in versus calories out. However, in practice it's more complicated than that, partially for psycho

At a really fundamental level, yes, it boils down to calories in versus calories out. However, in practice it's more complicated than that, partially for psychological reasons. Anyone will lose significant weight on a strict enough diet. Unfortunately, severe calorie restriction does things to your metabolism -- basically, your body notices there's not enough food, and tries to slow everything down so that your existing stores will last as long as possible. This (a) isn't good for you, since major organs are having to limp along, and (b) means your diet gets less effective, as "calories out" aggressively shrinks. There's evidence that this lasts for a fair while after a diet stops, making it really easy to regain weight. Then, psychologically, most people have a hard time sticking with any sort of long term diet. Unless you have an abnormally low hunger response, ignoring feeling hungry all the time (even just a little bit) is very hard. Cheating on your diet is very alluring, and your body gives you all the rewards for doing things that feel like they're avoiding you slowly starving to death. The latter point is where these drugs come in. By suppressing hunger signals, dieting suddenly becomes easy, removing the escalating willpower requirement. (Honestly, I'd worry about whether you'd still suffer the metabolic effects of calorie restriction even if you're not noticing the hunger, but hopefully that's being studied and they're only prescribed along with a solid nutritional plan...)