If a patient who is taking one of these drugs also takes St. John's wort, the drug may be broken down more quickly than usual, and it won't be fully effective. Dietary supplements may also interact with medicines that you can buy without a prescription, such as aspirin and cold remedies. In addition, some supplements may interact with alcohol. A few dietary supplements may also interfere with diagnostic tests. For example, the use of high-dose vitamine C supplements can interfere with tests for occult blood in the stool.
Some dietary supplements can have serious side effects even when taken alone.
Several herbs, such as chaparral, comfrey, and germander, can cause potentially fatal liver damage. Some Asian herbal remedies contain components that can cause severe kidney damage. Even vitamins and minerals can be harmful if taken in excessive doses. For example, overdoses of vitamin A can damage the liver. Taking too much vitamin D can damage the kidneys and cause calcium to be deposited in the soft tissues of the body.