I am a psychiatric pharmacist and see patients daily who deal with this problem. There really is no scientific rationale behind the weight gain, but there have been several proposed theories. From my experience, I've seen people gain and lose weight on SSRIs, so it really does depend on the individual. Paroxetine (Paxil), by far, has been shown to cause the most weight gain, so if weight gain is a big concern, I would steer away from that particular SSRI. Other medications that can cause weight gain include mirtazapine as well as most of the atypical antipsychotics (Seroquel, Zyprexa, Olanzapine). The sexual side effects and and stomach upset (nausea, diarrhea) associated with SSRIs can be attributed to the antagonism of the 5-HT receptors. We have these receptors all over our bodies and because SSRIs are not selective for the brain alone, we experience these side effects when the receptors are blocked in other parts of our body. The stomach upset will usually go away with time; however, the sexual side effects usually remain. If my patients are complaining about weight gain or sexual side effects, I usually recommend them to try other classes of antidepressants that have little antagonism of the serotonin receptors. Bupropion (Wellbutrin) and venlafaxine (Effexor) are just two examples that are usually well-tolerated without these side effects. Your doctor would have to be the one to evaluate whether these would be good options for you, but it never hurts to ask. Hope this helps.