My 17 yr old daughter is on acutane and has lost 13 pounds in two months, what kind of food can she eat keep her weight up? She pitches fast pitch softball and need to make sure she does not lose all her muscle and stamina. We would like her to gain weigh

Calorie dense foods. Eating calorie dense foods such as nuts, heavy cream, butter, fatty fishes, dried prunes, and avocado can help increase your daily calorie intake and assist in the desired weight gain. Supplements for weight gain would be inferior in quality to whole foods which are readily available and much less expensive.
I . I would discuss this at length with your dermatologist since losing that much weight seems concerning. Learn more about Accutane and some of the common issues encountered at the link below. I hope that helps!