How come antidepressants make me gain weight so much?

No, +appetite: Antidepressants don't make you gain weight directly, but serotonin is the neurotransmitter responsible for regulating satiety or "feeling full"; more serotonin in the synapses and the gut reduces satiety, so it takes more food to "feel full" and people tend to eat more. But with proper exercise and greater awareness of what one is eating, using antidepressants doesn't have to result in weight gain.

Agree w Dr. Tripp: It is important to know that some antidepressants are considered weight neutral (most people won't gain on them), some can assist with weight loss (like wellbutrin) and some can be associated with weight gain more often.