First pass metabolism

In this article in the series of ‘bite sized’ pharmacology, we will look at the concept of first pass metabolism. All drugs given by the oral route undergo a degree of first pass metabolism either in the gut or the liver, with some drugs being destroyed before they reach the systemic circulation. This pharmacokinetic process affects the bioavailability of drugs administered by this route and is an important consideration for the prescriber. Knowledge of first pass metabolism can assist the prescriber when deciding on doses and dose schedules to ensure that patients receive their medications at the correct dosing, by the correct route for optimum therapeutic effect. It also helps the prescriber understand why dose adjustments are made for some drugs when switching the route of administration from oral and why some drugs cannot be given by the oral route