Tadalafil is an oral drug and is used for the treatment of impotence. It is in a class of drugs
called phosphodiesterase inhibitors that also includes sildenafil (Viagra) and vardenafil (Levitra).
Erection of the penis is caused by the filling of the penis with blood.
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 that leads to an erection causes the production and release of nitric
oxide in the penis. The nitric oxide causes an enzyme, guanylate cyclase, to produce cyclic guanosine monophosphate (cGMP). It is the cGMP that is primarily responsible
for increasing and decreasing the size of the blood vessels carrying blood to and from the penis, respectively, and causing the erection. When the cGMP is destroyed by
another enzyme, phosphodiesterase-5, the blood vessels return to their normal size, blood leaves the penis, and the erection ends. Tadalafil prevents phosphodiesterase-5
from destroying cGMP so that cGMP stays around longer. The persistence of cGMP leads to a more prolonged engorgement of the penis with blood. Tadalafil was approved by
the FDA in November, 2003.

Recommended starting dose of tadalafil is 10 mg per day and should be taken before sexual activity. Depending on the adequacy of
the response or side effects, the dose may be increased to 20 mg or decreased to 5 mg a day. The effect of tadalafil may last up to 36 hours. Individuals who are taking
medications that increase the blood levels of tadalafil should not exceed a total dose of 10 mg in 72 hours (See drug interactions). It may be taken with or without food since
food does not affect its absorption from the intestine. Store this medication at room temperature between 15-30°C (59-86°F).

Most common side effects of tadalafil include
facial flushing (reddening), headaches, stomach upset, diarrhea, flu-like symptoms and nausea. Tadalafil also may cause low blood pressure, blurred vision and changes in
color vision, and abnormal ejaculation. Tadalafil has been associated with prolonged erections or priapism (painful erections lasting more than 6 hours). Patients should
seek immediate medical help if they experience an erection lasting more than 4 hours. Use of tadalafil, most often in patients with pre-existing heart disease, has been
associated with chest pain, heart attacks, death, strokes, palpitations and increased heart rate; however, it is not clear whether these side effects are due to tadalafil,
concurrent use of nitrates, or the sexual activity itself.

Tadalafil and alcohol both lower blood pressure. Therefore, combining tadalafil with alcohol may cause excessive
drops in blood pressure and cause dizziness, headaches and increased heart rate.
The breakdown and elimination of tadalafil from the body may be decreased by
erythromycin, ketoconazole (Nizoral), itraconazole (Sporanox), indinavir (Crixivan) and ritonavir (Norvir). Therefore, these drugs may increase the levels of tadalafil in the
blood. If these drugs are being used at the same time as tadalafil, the dose of tadalafil should be reduced in order to avoid side effects from high levels of
tadalafil.
Tadalafil exaggerates the increases in heart rate and lowering of blood pressure caused by nitrates, e.g. nitroglycerin, isosorbide dinitrate (Isordil), isosorbide
mononitrate (Imdur, Ismo, Monoket), nitroglycerin (Nitro-Dur, Transderm-Nitro) that are used primarily for treating heart pain (angina). In patients who take nitrates for
angina, tadalafil could cause heart pain or possibly even a heart attack by exaggerating the increase in heart rate and the lowering of blood pressure. Therefore, tadalafil
should not be used with nitrates.
Tadalafil also exaggerates the blood pressure lowering effects of some alpha-blocking drugs e.g., terazosin (Hytrin) that primarily are
used for treating high blood pressure or enlargement of the prostate. Individuals who take these alpha-blockers should be on a stable dose of the alpha-blocker before
tadalafil is started. In such situations, tadalafil should be started at the lowest dose. If the patient is already taking tadalafil, the alpha-blocker should be started at the
lowest dose.

Tadalafil is not approved for use in women.