"When considering taking supplements, it is important to recognize possible drawbacks that are specific to each kind. Some vitamins and minerals are toxic at high doses. Therefore, it is vital to adhere to the Tolerable Upper Intake Levels so as not to consume too much of any vitamin. For example, too much vitamin A is toxic to the liver and can cause tinnitus, blurred vision, hair loss, and skin rash. Too much niacin can cause a peptic ulcer, hyperglycemia, dizziness, and gout. Some herbs cause side effects, such as heart palpitations and high blood pressure, and must be taken very carefully. Some herbs also have contraindications with certain medicines. For example, Valerian and St. John’s Wort negatively interact with certain prescription medications, most notably antidepressants. There is also a real risk of overdosing on herbs because they do not come with warning labels or package inserts. Certain amino acid supplements, which are often taken by bodybuilders among others, can increase the risk of consuming too much protein. An occasional amino acid drink in place of a meal is not a problem. However, problems may arise if the supplement is added to an existing diet. Most Americans receive two to three times the amount of protein required daily from their existing diets, so taking amino acid supplements just adds to the excess. Certain amino acids share the same transport systems in the absorption process; therefore, a concentrated excess of one amino acid may decrease absorption of another amino acid that uses the same transport system, potentially leading to deficiency in the competing amino acid."