Every medicine has side effects. Slight weight gain is one of the side effect of contraceptive pills but it's not that significant as it is used to be. Weight gain is due to water retention effect of hormone (estrogen) in contraceptive pill. This effect is seen usually in large dose of estrogen which is much lower now a days. So weight gain effect is minimum to nil. If anyone experiences weight gain, can take another form of contraceptive pill that is devoid of estrogen.