What does 'first pass effect' refer to in drug metabolism?

The term 'first pass effect' describes the initial metabolism of a drug that occurs in the liver before the drug enters systemic circulation. When a drug is administered orally, it is absorbed from the gastrointestinal tract and travels through the portal vein to the liver. During this process, a portion of the drug may undergo significant biotransformation, which can reduce the concentration of the active drug that reaches the systemic circulation. This phenomenon is particularly important for drugs that are highly metabolized by the liver, as it can greatly influence their bioavailability and therapeutic effectiveness.

If a drug has a high first pass effect, it may require a higher oral dosage to achieve the desired therapeutic effect compared to routes of administration that bypass this metabolic step, such as intravenous injection. Understanding the first pass effect is crucial for healthcare professionals when determining dosages and routes of administration for medications to ensure efficacy and safety for patients.