First pass metabolism is when a consumed drug loses the majority of its concentration before it reaches the circulatory system, and thus negates the intended effect of the drug. Other names for first pass metabolism are "first pass effect" and "presystemic elimination."
All drugs must enter through the liver before entering through the rest of the body, and so most of the time first-pass metabolism occurs in the liver. The intestine can also serve as a site for first-pass metabolism.
Drugs that are susceptible to first pass metabolism must be administered in a different fashion. For instance, they can be directly injected or, if the drug is susceptible to first-pass metabolism in the mouth, it can be given as an enema. In some cases, first-pass metabolism is preferred in order to naturally reduce the strength of the drug.
Several common drugs are affected by first-pass metabolism. These include aspirin, enalapril, oxprenolol, prazepam, glyceryl trinitrate and amitriptyline.