A. Brittany Loriquet, a dietetic intern in Tufts Frances Stern Nutrition Center, explains: Vitamins can be derived from food products or produced synthetically in a lab. Most multivitamins are made from synthetic vitamins, which are cheaper and easier to use than those from natural foods. There is no difference in the chemical structure between the synthetic form and naturally derived forms. To make a multivitamin, the vitamins and minerals are ground into a fine powder.
Multivitamins usually contain additives that help in the manufacturing process or how the pill is used by the body. These additives may serve as fillers to provide the multivitamin with bulk, flow agents to help the powder mixture run through machinery, or disintegration agents that help the pill break up after ingestion.
The powder mixture and the additives are run through machinery that either compresses them into tablets or encapsulates them in gelatin capsules. Many tablets are coated for a variety of reasons, including changing the flavor, making the pill easier to swallow, or determining where (stomach or small intestine) and how quickly (slowly over time or all at once) the pill will be absorbed.
After you swallow a multivitamin, the body treats it in the same way as any other food or drink. The pill will travel through the gastrointestinal tract. Most vitamins and minerals are absorbed in the small intestine. They will enter the bloodstream and ultimately end up where they are needed in the body (this is highly variable, since each vitamin and mineral has a distinct function). Anything that is not needed by the body will be excreted.