Vitamin E is a fat-soluble vitamin with antioxidant properties. Vitamin E exists in eight different forms ("isomers"): alpha, beta, gamma, and delta tocopherol; and alpha, beta, gamma, and delta tocotrienol. Alpha-tocopherol is the most active form in humans. Dosing and daily allowance recommendations for vitamin E are often provided in Alpha-Tocopherol Equivalents (ATE) to account for the different biological activities of the various forms of vitamin E, or in International Units (IU), which food and supplement labels may use. Vitamin E supplements are available in natural or synthetic forms. The natural forms are usually labeled with the letter "d" (for example, d-gamma-tocopherol), whereas synthetic forms are labeled "dl" (for example, dl-alpha-tocopherol). Vitamin E has been proposed for the prevention or treatment of numerous health conditions, often based on its antioxidant properties. However, aside from the treatment of vitamin E deficiency (which is rare), there are no clearly proven medicinal uses of vitamin E supplementation beyond the recommended daily allowance. There is ongoing research in numerous diseases, particularly in cancer and heart disease. Recent concerns have been raised about the safety of vitamin E supplementation, particularly in high doses. An increased risk of bleeding has been proposed, particularly in patients taking blood-thinning agents such as warfarin, heparin, or aspirin, and in patients with vitamin K deficiency. Recent evidence suggests that regular use of high-dose vitamin E supplements may increase the risk of death (from "all causes") by a small amount, although a different study found no effects on mortality in women who took vitamin E daily. Caution is warranted.