The pharmacokinetic journey of oral drugs begins with a crucial first pass through the hepatic portal system, called the first-pass effect. This first pass significantly impacts bioavailability — the proportion of a drug that enters systemic circulation and is available for therapeutic action. The primary route sees the drug absorbed by intestinal membranes and then shunted to the liver via the hepatic portal vein. Here, pre-systemic elimination occurs as drugs face metabolism or biliary excretion, diminishing their bioavailability.

The extent of hepatic or first-pass metabolism is quantified as the liver extraction ratio (ER), which is the fraction of the drug removed during its first pass through the liver. It is dictated by hepatic blood clearance rates and circulatory dynamics. Drugs such as theophylline have low extraction ratios and undergo minimal pre-systemic elimination. As a result, such drugs are completely absorbed post-oral administration. In contrast, drugs with high ER, such as verapamil, propranolol, and nitroglycerin, are extensively metabolized pre-systemically. This leads to poor bioavailability and higher dose requirements to achieve desired therapeutic effect.

Alternative administration pathways, such as sublingual or transdermal, are employed to subvert this pre-systemic elimination. For example, 90% of oral nitroglycerin is metabolized pre-systemically. To improve its bioavailability, it is often administered sublingually, which bypasses the pre-systemic metabolism and preserves its potency and therapeutic efficacy.