I don't know if it causes weight gain, but if there are lots of stuff online about it, you should research it.

Here's why meds cause either weight gain or loss. The med changes the body's weight set-point by telling the brain to either gain or lose weight to the new set-point so you gain or lose regardless of what you eat. A good example of this is how women used to use cigarettes to control their weight. Most people gained 10 lbs or more after they stopped smoking.