It should work for both. The booklet that comes with the shake talks about losing, maintaining, and gaining weight. Basically you need to figure out your calories needs and, in order to gain weight, go above. I have 3 shakes a day (1500 calories) because I need to lose weight (I’m doing 100% shakes most days, which I know not everyone does). For you, use a calorie calculator and see how many shakes you want/need based on your goals. Adding an extra 500 calories (one shake) to your normal intake should help with weight gain.