Can 15mg of hydrocortisone cause weight gain?

Yes: Side effects associated with oral steroids include but are not limited to: mood swings, irritation of the stomach, weight gain, gastrointestinal intolerance, and increased blood sugars (if diabetic). Weight gain with steroids is mainly water weight and will be lost once stopped, however steroids make you hungry so if you eat a lot of unhealthy food, that weight will be more difficult to loose.