I have gained a lot of weight on celexa (citalopram). does that cause weight gain and how can i lose it?

It can happen: Weight gain is a possible side effect with celexa. You might talk to your physician about the possibility of changing from Celexa to it's sister drug- lexapro (escitalopram). It may be more weight neutral for you. But, the only way to get the extra weight off is the good old fashioned way: exercise and ensuring that calories burned exceeds caloric intake. Good luck.