My son has been on meds since he was 5 he was always underweight, any of the medications we switched him to affected his appetite. He just would not eat, and when he did he just picked at his food. It’s been ten years and he’s finally at a good weight and has been for a couple of years, what his ADHD doctor suggested was high calorie shakes. So for lunch instead of school lunch or sending food he wouldn’t eat we sent a protein powder mixed with Carnation instant breakfast in a shaker bottle all he had to do was pour in the milk and he was set. He loves it. He managed to increase weight and maintain it. With him being a teenager now he does eat more but usually at home after the medicine has worn off.