Lamictal and Haldol are my mix and neither have caused weight gain for me. As a mood stabilizer, I've rarely, if ever, heard of weight gain with Lamictal. Haldol, like most antipsychotics, all depends on the person. Pretty much all APs run the potential risk of weight gain, but this is not universal. Some of the newer APs (like Abilify and Rexulti) seem to be statistically less likely to cause it. But, again, you never know what will or won't affect you. As an old AP, Haldol is known to have a high incidence of side effects like weight gain, but it hasn't done that to me at all. Whether or not weight gain occurs, it's always beneficial to watch one's diet on these kinds of meds, if only to aid blood sugar.