The hydrochloride salt of tamsulosin, a sulfonamide derivative with adrenergic antagonist activity. Tamsulosin selectivity binds to and blocks the activity of alpha1 adrenoreceptors in the human prostate and bladder neck; blockade of these adrenoceptors can cause smooth muscle in the prostate and bladder neck to relax, resulting in an improvement in urinary flow rate. or A drug used to treat urinary problems caused by an enlarged prostate. Tamsulosin hydrochloride relaxes the muscles of the prostate and bladder, which helps the flow of urine. It is a type of alpha blocker. Also called Flomax and tamsulosin.