People often mistakenly assume that because supplements are sold over the counter, they are completely safe to take, even in high doses. For example, even though no studies have proven that large doses of vitamin C can prevent or cure colds, many people still think this is true. You may hear claims about other benefits of taking large doses of certain vitamins. But using large doses of vitamins to fight disease in humans is not supported by scientific evidence so far.
In fact, large doses of some vitamins or minerals have been shown to be dangerous. For example, too much vitamin C can interfere with the body’s ability to absorb copper, a metal that’s needed by the body. Too much phosphorous can inhibit the body’s absorption of calcium. The body cannot get rid of large doses of vitamins A, D, E and K, and these can reach toxic levels when too much is taken.
Talk with your doctor or pharmacist about all the supplements you take, along with all the medications you take, to ensure that you are not at risk for dangerous interactions and to make sure you are taking the correct dose.
— Source: American Cancer Society