The amount of barberry taken depends on the condition for which it is being used. Most health practitioners recommend that you do not take barberry for more than 5-7 days in a row. If you have a sensitive stomach, you should limit it to 3-5 days. Barberry is safe when taken as directed. However, taking large doses for an extended period of time may irritate your stomach and make it difficult for the body to absorb B vitamins.
Excessive doses can cause lethargy, nose bleeds, nausea and diarrhea, among other symptoms.
The American Herbal Products Association has given barberry a safety rating of 2B. It should not be used by pregnant women. A Handbook of Bioactive Compounds from Plants . Ivanovska N, Philipov S. Study on the antiinflammatory action of berberis vulgaris root extract, alkaloid fractions, and pure alkaloids. Rodale's Illustrated Encyclopedia of Herbs. McGuffin M, Hobbs C, Upton R, Goldberg A. American Herbal Products Associations's Botanical Safety Handbook.