Tadalafil (also known as Cialis) is a phosphodiesterase-5 (PDE5) inhibitor used mainly for treating erectile dysfunction and benign prostatic hyperplasia (BPH). An erection is caused by blood filling the penis. This filling occurs because the blood vessels that bring blood to the penis increase in size and deliver more blood to the penis, and, at the same time, the blood vessels that take blood away from the penis decrease in size and remove less blood from the penis. Sexual stimulation causes the production and release of nitric oxide in the penis, leading to an erection. Nitric oxide causes an enzyme, guanylate cyclase, to produce cyclic guanosine monophosphate (cGMP). It is cGMP that is primarily responsible for increasing and decreasing the size of blood vessels carrying blood to and from the penis, respectively, and causing an erection. Another enzyme named phosphodiesterase-5 destroys cGMP, causing the blood vessels to return to their normal size, which allows more blood to leave the penis, ending an erection. Tadalafil prevents phosphodiesterase-5 from destroying cGMP, allowing the elevated level of cGMP to persist for a longer duration. The persistence of cGMP leads to a more prolonged engorgement of the penis with blood. Tadalafil is much longer-lasting than some of the other popular drugs in its class, such as Sildenafil (Viagra). The effects of Tadalafil can be felt between 36 and 48 hours. 

The mechanism by which tadalafil improves the symptoms of BPH is not clear. But, interestingly, phosphodiesterase-5 is also present in the muscles of the bladder and the prostate. As such, it has been suggested that the relaxation of these muscles may make the passage of urine less difficult by reducing the pressure in the muscle surrounding the opening to the urethra that controls the flow of urine from the bladder. The FDA approved Tadalfil in November 2003 for the treatment of erectile dysfunction and in 2011 for the treatment of BPH. 

WARNING: Do not take Tados if you are also using a nitrate drug for chest pain or heart problems. It can cause a sudden and serious decrease in blood pressure. This includes nitroglycerin, isosorbide dinitrate, isosorbide mononitrate, and some recreational drugs such as amyl nitrate or nitrite ("poppers"), which contain nitrates. Taking tadalafil with a nitrate medicine can cause a sudden and dangerous decrease in blood pressure.