How long does it take for a vitamin to start working?

While the amount of time it takes for a vitamin to start working depends on the type of vitamin and style of pill, most vitamins need to be processed in the stomach before moving to the small intestine, which can take up to 3 or 4 hours, according to NutriStrategy. Once in the small intestine, the vitamin will be absorbed within minutes.

Vitamins that are water-soluble, such as B vitamins, will be absorbed significantly quicker than fat-soluble vitamins such as vitamin K. After the vitamin gets absorbed by the small intestine, it can start working immediately to aid the body in biological processes, notes HelpGuide.