Deprovera

I am 22 and I have been on the shot for about 2 1/2 years. I have had a lot of mood swings from, this and lost my sex drive for the first year. I have not had one period since i have been on it (loving that). However, i have never experienced any weight gain. I have always been really skinny and was hoping to gain a few pounds by taking the shot. Over all i like being on the shot, mainly because of not having a period. Im just wondering how long i can stay on the shot.