Just make sure that, if you're aircrew (I know there are some people on this forum who aren't, so disregard if you're not going into the air) that what you are taking is within the rules. You can get taken off duty for taking the wrong stuff. When I was stashed, my job was to help research dietary supplements sold on base, and almost every product sold at places like GNC contains at least one Class C (prohibited at all times) substance or ingredient.
While Desertoasis is correct, no one actually seems to give a damn. When I was in flight school, the doc approved amino acids I was taking. I made sure to double check with the flight doc for each new command and it was never an issue. I was in a Class A mishap in March, and a flight doc not associated with the squadron asked if I was taking any supplements. I said, yes, amino acids. His response was "You know you are not allowed to take those right?" I responded with "No I didn't know that, otherwise I wouldn't have said anything." It was left at that. I had blood/urine tests to satisfy the mishap rules and it was never mentioned again.