An excipient is a substance used as a "filling agent" in a medication. In general, the active substances (such as aspirin) may not be easily administered and absorbed by the human body; they need to be put in some appropriate form. The active substance is then dissolved or mixed with an excipient.

Depending on the route of administration, and medication form, various excipients may be used:

• oral route
  • syrups, consisting of an excipient of a mix of sugar and water
  • tablets, using excipients such as sugar or corn starch
• rectal route
  • suppositories made of a greasy substance such as cocoa butter

Once the active ingredient has been purified, it cannot stay in purified form for very long. In many cases it will denature, fall out of solution, or stick to the sides of the container. To stabilize the active ingredient, excipients will be added to ensure that the active ingredient stays active, and is stable for a long period of time to ensure that the shelf-life of the product makes it competitive with other products. The formulation of these excipients in many cases is considered a trade secret.