Many medications used to treat mental illness cause weight gain in many users. Different meds have different effects, so while one might result in weight gain, others won't. It's trial and error. When the only drug that works causes a lot of weight gain, that's a big problem. With antidepressants, there are ones out there for depression that don't have a record of causing weight gain, but they tend to be stimulating drugs and for some this will mean increased anxiety, while for others it won't. The only thing you can do is wait and see if this stops assuming he's only been on it for a short period of time. I can't really really tell you if an injected form would be different, only that since an injected drug would presumably be better absorbed it would more likely just intensify whatever effects one is having, whether beneficial or negative. I would also assume the current form is probably less well absorbed than a regular pill. What causes the weight gain is up for some debate, but most likely it's the sedating effect of most drugs available to fight mental illness. Some have offered that a change of diet and exercise can help with this, but my own experience and that of most people is this doesn't really do much. I would guess that if this is a permanent side effect then the only option is to look for another medication that doesn't have this side effect but also works. It's hard to stop a drug that is working, though. You don't say how old he is or how much he moves or what he eats -- if his diet is really bad and he lives an entirely sedentary life, then it's more possible a better diet and a large increase in exercise and movement can help. If his diet is okay and he isn't sedentary, that's a lot harder to imagine changes in that area will help. In my case, I haven't taken antipsychotics, only antidepressants, and only one of several I've taken caused the weight gain, and I was someone who ate very well and exercised a lot. It was the drug entirely. Sometimes it just works that way.