Could drinking a protein shake cause constipation? Here’s what our in-house registered nutritionist, Holly Roper, says: “Protein shakes affect everyone differently, but usually, they only cause constipation if you have them in excessive quantities or you don’t drink enough water. If you feel that your protein shake is leaving you feeling uncomfortable and unable to go to the toilet, we recommend that you stop taking it. If the symptoms continue, it’s important to speak to your GP.”