Hi, welcom to HCM.
Weight gain can be due to various reasons such as lack of activity, hormonal
problems, overeating etc. Corrections should be made in the lifestyle before you can go on any medication. The weight tends to come back when you stop the medication and you tend to continue the medication for a longer period which might have other effects in the later years. Since you are still young, its better to try weight loss
through diet and exercise rather than medication. Take up a regular exercise at least for 30 minutes a day and also consume a healthy balanced diet with a lesser portion size and increase the consumtion of fruits and vegetables. Avoid fatty foods, fried foods and sweets. Consume at least 3-4 litres of warm water everyday. Diet and exercise can help you in a longer run.