An opiate alkaloid isolated from the plant Papaver somniferum and produced synthetically. As a direct-acting smooth muscle relaxant, papaverine is not closely related to the other opium alkaloids in structure or pharmacological actions; its mechanism of action may involve the non-selective inhibition of phosphodiesterases and direct inhibition of calcium channels. This agent also exhibits antiviral activity against respiratory syncytial virus, cytomegalovirus, and HIV.