First-Pass Effect

Excerpt

The first-pass effect is a pharmacological phenomenon in which a medication undergoes metabolism at a specific location in the body. The first-pass effect decreases the active drug's concentration upon reaching systemic circulation or its site of action. The first-pass effect is often associated with the liver, a major site of drug metabolism. However, the first-pass effect can also occur in the body's lungs, vasculature, gastrointestinal tract, and other metabolically active tissues. This effect can be increased by various factors such as plasma protein concentrations, gastrointestinal motility, and enzymatic activity. The extent to which a patient may undergo the first-pass effect differs from patient to patient, and this must also be considered when determining appropriate dosing. If the first-pass effect is significantly prominent in a patient, the medication may require administration via a different route or formulation to bypass the first-pass effect.