Am 21 years old but my weight is only 40 kg's and skinny.can you please suggest me diet or any protein powder which doesn't have any side effects.....

Studies first: At that weight you should first see your doctor to get a physical and boloodwork/ studies then evaluate nutritional status.

Gain muscle?: Do you want to gain muscle or gain fat or a reasonable amount of both? Protein powder isnt the answer. You need to take in more calories than presently and i suggest it from healthy foods, like nuts- whole grains etc- you also need to challenge your body to increase the muscle through exercise- resistance training.

Agree w Doctors: Fried and waskowitz. Foods to help with healthy weight gain: almonds, cashews, pecans, sunflower seeds, flax seeds, walnuts, hummus, avocadoes, granola, trail mix, dried fruits, extra virgin olive oil, canola oil, protein shakes, protein bars, lean meats, poultry, fish, smoothies (with added silken tofu or powdered milk), greek yogurt (full –fat), pasta/ bagels ; use of juice rather than soda.