Tadalafil is a selective phosphodiesterase type 5 (PDE5) inhibitor. Inhibition of PDE5 increases cGMP in smooth muscle cells. cGMP causes smooth muscle relaxation and increased blood flow into the corpus cavernosum, causing penile erection. PDE5 also is present in smooth muscles of the prostate and bladder wall. Inhibiting PDE5 increases cGMP concentrations leading to relaxation of smooth muscle in the prostate and bladder. Smooth muscle relaxation may improve blood flow to the urinary tract and widen the opening of the bladder neck, resulting in improved voiding.