Casein supplementation is safe for most people. It is a dairy product, so people who do not tolerate dairy may experience mild gastrointestinal side effects, especially bloating and flatulence, when using casein. These symptoms could point to being intolerant to casein proteins themselves, or they could indicate lactose intolerance.[1]
A casein allergy is also possible. This is an inappropriate immune reaction to the proteins in casein.[2] Although avoiding casein would be prudent in the case of casein intolerance or allergy, an allergic reaction is more severe and can be life-threatening.