Why do birth control pills side effects say weight gain but some doctors say only the shot ...but plenty of women complain about bc pill weight gain?

Varies: The pill will cause some cyclic variation in fluid retention. That would appear to cause weight gain. That fluid retention would wax and wane with the cycle, so the gain would wax and wane as well with no cumulative net gain over time. Women who gain weight over time are ingesting more calories than they are burning and those calories do not come from the birth control pill.