01.06 Sublingual

Extensive first pass liver metabolism makes it difficult to administer some drugs by the oral route. Sublingual (under the tough) administration can be used to avoid first pass liver metabolism. Drugs that are absorbed via the oral mucosa drain into the superior venae cavae to the heart and systemic circulation; they are not transported directly to the liver, and thus do not undergo first pass liver metabolism. Once in the systemic circulation, only part of the drug will pass through the liver with each circulation - the rest will go through other organs or tissues, which may not metabolise the drug to destroy it: Figure 1.5.

Figure 1.5 Sublingual drug administration (Copyright QUT University, Sheila Doggrell)

The sublingual route of administration increases the bioavailability of drugs that are extensively metabolised by the liver. The best known example of a drug that is used sublingually to avoid first pass liver metabolism is glyceryl trinitrate (nitroglycerin). Oral nitroglycerin is ineffective as it is extensively metabolised by the liver, and little enters the circulation if taken orally. In an attack of angina, nitroglycerin administered sublingually does get into the circulation, and rapidly provides relief from the attack of angina.