To safely gain weight and build muscle, the most effective and thoroughly researched supplements are whey protein and creatine monohydrate. Whey protein helps you meet your daily protein targets for muscle repair, while creatine safely increases strength and muscle volume. Both are highly safe with virtually no side effects when used in recommended doses, though you must drink plenty of water. Remember, supplements only work alongside a caloric surplus (eating more calories than you burn) and consistent strength training. Always consult your doctor before starting any new supplement regimen.