First Pass Effect

Orally administered drugs must pass into the hepatoportal circulation (liver) before entering the general circulation. The first-pass effect is a process whereby only a fraction of an orally administered drug reaches systemic circulation. 

Much of the drug is metabolized in the liver to an inactive metabolite before passing into the general circulation. 

 First Pass Effect

Orally administered nitroglycerin is approximately 90% cleared during a single pass through the liver. Sublingual administration avoids the first-pass effect. Many blood vessels are located under the tongue, so drugs.

administered by this route can pass directly into the general circulation before passing through the liver. Morphine is also subject to the first-pass effect.