The mesylate salt form of doxazosin, a quinazoline compound with smooth muscle relaxing activity. Doxazosin mesylate selectively antagonizes alpha-1-adrenergic receptors in smooth muscle of the bladder neck and prostate, thereby relaxing the smooth muscle and decreasing the obstruction and urethral resistance seen with benign prostate hyperplasia (BPH). This may improve BPH symptoms. This agent also blocks alpha-1-adrenergic receptors in peripheral vascular smooth muscle, which leads to vasodilatation and a subsequent decrease in peripheral vascular resistance.