I see many people in my practice that take multiple vitamins in large quantities and doses. There is an overwhelming culture fueled by commercials and urban legends that more is better when it comes to supplements. Nothing could be further from the truth when it comes to vitamins.
Vitamins are needed as a cofactor in metabolic reactions in our body. Only a small amount is needed to move the metabolism forward. An example is in order. Think of a fireplace stacked with wood and kindling ready to go; you need only one match to start the fire. Two, three, four matches do not increase the intensity of the fire.
We need only minute amounts of vitamins to keep our metabolic rates moving along. A normal well balanced diet supplies the vitamins in the quantity that is necessary. The excess dosages that we take in are eliminated by our liver and kidneys. Some vitamins like E when taken in huge amounts (1000 units vs the needed 35 units/day) overwhelm our finely tuned system and are stored in fat cells only to be released slowly affecting normal platelet function and causing excessive bleeding during surgery. Vitamin D when ingested in very high doses can lead to serious illness including death.
Our bodies are usually well self regulated through a very complex system of organic reactions that are exquisitely balanced and do not need our pharmaceutical input.