i want to gain wieght

Concerta may cause nausea, vomiting and loss of appetite. Ask your doctor if it is alright to use over the counter anti-nausea medication. Eating multiple small meals over the course of the day will get you the nutrition that you need to gain weight.

I agree that eating more small meals during the day should help you out. You may also want to consider eating more high calorie foods that are not necessarily bad for you.

For example, you could try drinking a few Slim Fast shakes on top of the meals that you eat to help you slowly gain a bit more weight. Please let us know how things turn out.