Excipients are inactive substances used as a carrier for the active pharmaceutical ingredient (API) of a medication.
Excipients often contribute huge functionality to the product. In solid oral dosage forms such as tablets, excipients hold the tablet together as well as allow it to break apart in a controlled and well-understood way in the digestive system. They can mask unpleasant tastes as well as preserve shelf life. In other pharmaceutical formulations, such as topical preparations or suppositories, excipients help to keep the product components homogeneously suspended, ensure even spreading, and protect against drying out or viscosity changes that might affect delivery of the therapeutic agent.