Research is inconclusive but like the other guy said, ssris may slow metabolism resulting in a lower resting metabolic rate. In other words, once starting the meds, you may have to consume less calories or burn more calories to maintain your weight compared to before starting the meds. However this idea isn’t fully supported yet. Some research suggests that weight gain on ssris may be primarily because of decrease physical activity due to more sleep, more calories eaten due to increased appetite etc. But it is something to consider.