Opioids: Lung first pass

The lung has the unique ability to retain certain medications after they are administered intravenously. When intravenous drugs are taken up by the lungs, they can slowly be released back into the bloodstream as systemic concentrations fall, which can help maintain a steady concentration of drug. This is called first pass pulmonary uptake. First pass pulmonary uptake is particularly important and significant when speaking of lipophilic amines (lidocaine, propranolol and some opioids).

Specifically regarding the opioids, meperidine, fentanyl, sufentanil and alfentanil will be retained by the lung in significant amounts after IV administration (first-pass uptake). Morphine does not undergo lung first pass metabolism.