Benign prostatic hyperplasia
In men the most common cause of urinary retention is benign prostatic hyperplasia (BPH), due to the blocking of flow of urine out of the bladder. There are three classes of drugs used to treat BPH:
Alpha1 receptor antagonists (alpha blockers)
Doxazosin and prazosin are alpha blockers that act by relaxing the smooth muscle at the opening of the urethra, increasing the flow of urine into the bladder. These drugs are generally well tolerated but can cause postural hypotension.
Bethanechol is a muscarinic receptor agonist that causes detrusor muscle contraction and the effect of this drug is most marked when there is bladder outlet obstruction, but it has no role in the relief of acute urinary retention. Muscarinic receptor agonists can produce a number of side effects including sweating, bradycardia and intestinal colic.
Finasteride is a specific inhibitor of 5 alpha reductase, an enzyme that converts testosterone to the more potent androgen, dihydrotestosterone, which leads to a reduction in prostate size and thus improves the flow of urine.