Popping vitamins may sound like a good way to assist your immune system. But if you have hepatitis C, a viral disease that leads to inflammation of the liver, some supplements might actually slow you down.

According to Edward Block, MD, a board-certified internist and gastroenterologist, nutritional supplements (when taken in moderation) are usually good for your body—they’re designed to boost low levels of vitamins and minerals, including iron—but some supplements can be toxic to the one organ already under attack from hep C, your liver.

Why? Because individuals with hepatitis C, especially those with an abnormal liver condition called cirrhosis, already have higher than usual amounts of iron in their bodies, and that usually safe supplement may boost their levels into a danger zone, ultimately leading to organ damage.

So before you dash to the health food store, check with your doctor and make sure those good-for-you vitamins and minerals are actually doing your liver a favor.