I haven't heard about the temperature regulation side effect, maybe that's something to comment to your doctor about, but I've certainly heard it (mirtazapine) can cause weight gain. I've been taking mirtazapine for years now (10+ years) at either 45 mg or 30 mg and I can safely say it has been a good medicine for me. I take 30 mg right now. It didn't cause weight gain for me but for some it can, and it can be problematic to some people. Bupropion is a medication I'm a little less familiar with, but perhaps the combination of it and mirtazapine will mitigate weight gain? I say that from a complete layperson's perspective, I am no doctor (nor do I plan to be), but I do know a fair amount about these types of medicines because I've been on them for 15+ years. Anyway, I think if it's concerning enough, the temperature regulation thing should be commented to by you to your doctor, and perhaps you could relate the concern for weight gain as well at that consultation? What do you think?