I've taken MRPs, creatine, glutamine, and vitamins on international flights without any problems.
One time when I was coming back from Spain a woman checking the contents of carryons gave my plastic shaker cup a thorough going over. Apparently she had never seen one of those before because she called a supervisor over. The supervisor looked at it, and made a drinking motion while looking in my direction. I nodded and then he laughed and returned the cup to me.