Can Casein Protein Powder cause diarrhea?

Yes, casein protein powder contains lactose and can cause digestive issues for those with some lactose intolerance, including diarrhea. Those allergic to milk products can also experience diarrhea after consuming casein protein powder. If you suffer from lactose intolerance of an allergy to milk products you can use egg protein powder as an alternative. It is not quite as effective, but offers a similarly slow digesting protein.