Papaverine

Papaverine is a vasodilatory and smooth muscle–relaxant alkaloid derived from the opium poppy Papaver somniferum. It is a benzylisoquinoline alkaloid and is distinct from the morphine-family compounds.

Mechanism of action: Papaverine acts mainly by inhibiting phosphodiesterases and by blocking calcium influx in smooth muscle cells. This increases intracellular levels of cyclic AMP and cyclic GMP, leading to relaxation of vascular and visceral smooth muscle. It is non-narcotic and does not act on opioid receptors.

Medical uses: Historically, papaverine has been used to relieve spasms of the biliary, gastrointestinal, and genitourinary tracts. It has also been employed as a vasodilator to counteract arterial spasm in the brain after subarachnoid hemorrhage and in various intraoperative vascular settings. In urology, intracavernosal injections of papaverine (often in combination with other agents) have been used to treat erectile dysfunction.

Administration and pharmacokinetics: Papaverine is available in oral, injectable, and topical forms, with systemic doses limited by the potential for hypotension. Pharmacokinetics depend on the route of administration; it is metabolized by the liver and excreted in the urine.

Safety and adverse effects: Common adverse effects include hypotension, flushing, dizziness, and headache. Local injections can cause pain and, with penile use, priapism. Coadministration with other vasodilators can increase the risk of hypotension; caution is advised in patients with low blood pressure or significant cardiac disease.

History and status: Papaverine was isolated in the 19th century from opium. It remains in use in certain niche clinical contexts, though many indications have been supplanted by newer therapies.