The reason that antidepressants "cause" weight gain is because a lot of people that are depressed aren't really interested in food, so they don't eat as much.  After the medicine starts to work, they fell better and start eating more.  More food means weight gain.  The medicine itself didn't cause the weight gain, the change in behavior did. This is actually true of a lot of medications.  They alter a person's interest in food and therefore the amount they eat.  Amphetamines themselves don't really cause weight loss, but they curb the appetite, resulting in eating less.  It's all a matter of self control.