The first pass effect is a phenomenon in which a drug gets metabolized at a specific location in the body that results in a reduced concentration of the active drug upon reaching its site of action or the systemic circulation. The first pass effect is often associated with the liver, as this is a major site of drug metabolism. However, the first pass effect can also occur in the lungs, vasculature, gastrointestinal tract, and other metabolically active tissues in the body. This effect can become augmented by various factors such as plasma protein concentrations, enzymatic activity, and gastrointestinal motility.[1][2][3][4] The extent to which a patient may experience the first pass effect varies from patient to patient, and this must also be taken into consideration when determining appropriate dosing.[5][1] If the first-pass effect is exceptionally prominent in a patient, the drug may require administration via a different route to bypass the first-pass effect.[6]

A significant issue of concern with the first pass effect is taking into account its variability among different individual patients. It is incredibly important that pharmacological dosing considers these natural variations in human metabolism to ensure patients remain within the therapeutic window of the appropriate drug.[5] Also, there have been noted differences in the first pass effect based on gender and age.[7][8]

The clinical significance of the first pass effect is crucial to the proper administration and maintenance of pharmacological therapy. Some drugs that undergo considerable first-pass metabolism include alprenolol, 5-fluorouracil, morphine, pentazocine, and mercaptopurine. When given orally, these drugs are quickly metabolized via the first-pass effect, requiring their oral dosages to be much larger than their intravenous dosages. The first pass effect also has an impact on peak drug concentrations, which may result in drug concentration peaks occurring much earlier than they would in a parenteral dose.[1][9]

It is critical to maintain proper serum concentrations of a drug that experiences the first-pass effect; this allows for the maintenance of a safe and effective dose of the drug. Research has shown that monitoring blood levels of drugs that experience the first-pass effect is the most viable way to maintain therapeutic concentrations of these drugs.[10] [Level 3]

When monitoring patients that are taking drugs that experience the first-pass effect, it is critical to monitor the blood levels of these drugs to ensure that the patients' serum drug concentrations remain within their therapeutic windows. Doing so will maximize the efficacy of treatment and patient safety.[10]