Supplements are no substitute for a healthy diet and, in some circumstances, can even be dangerous. Certain vitamins taken in large quantities can be toxic. These include vitamins A, D, E and K. People taking the blood thinner warfarin need to particularly be careful about the amount of vitamin K they consume. In addition, some supplements can interact with other cardiac medications, especially digoxin. The role of antioxidant vitamins, especially vitamins C and E, remains somewhat controversial.
Some studies show beneficial effects, and some studies show no effect or detrimental effects.
In general, you should check with your health care provider before beginning any supplement, and you should remember to tell health care providers about what supplements you take so they can help you avoid interactions.