Penile erection is caused by the engorgement of the penis with blood. This engorgement occurs when the blood vessels delivering blood to the penis increase in size and increase the delivery of blood to the penis. At the same time, the blood vessels carrying blood away from the penis decrease in size and decrease the removal of blood from the penis. Sexual stimulation that leads to the engorgement and erection causes the production and release of nitric oxide in the penis. Nitric oxide then activates the enzyme, guanylate cyclase to produce cyclic guanosine monophosphate (cGMP). The cGMP is primarily responsible for increasing and decreasing the size of the blood vessels carrying blood to and from the penis, respectively.

Tadalafil prevents an enzyme called phosphodiesterase-5 from destroying cGMP so that cGMP persists longer. The longer cGMP persists, the more prolonged the engorgement of the penis.

Tadalafil was approved by the FDA in November, 2003.

Tadalafil doses
For most individuals, the recommended starting Tadalafil dose is 10 mg per day taken before sexual activity. Depending on the adequacy of the response or side effects of Tadalafil, the Tadalafil 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 should not exceed 10 mg in 72 hours.

Storage instructions for Tadalafil: Store at room temperature between 15-30°C (59-86°F).

Preparation of Tadalafil Tablets: Tadalafil 5 mg, Tadalafil 10 mg and Tadalafil 20 mg.

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 concentration 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 increases the heart rate and also exaggerates the blood pressure lowering effects of nitrates (e.g., nitroglycerin). In patients who take nitrates for chest pain (angina), could cause chest pain by increasing heart rate and lowering blood pressure. Therefore, patients with angina should not use Tadalafil.

Tadalafil also exaggerates the blood pressure lowering effects of some alpha-blocking drugs e.g., terazosin (Hytrin). Individuals who take these alpha-blockers should not use Tadalafil .

Tadalafil is not approved for use in pregnant women. Tadalafil has not been evaluated in women who are breastfeeding.

The most common Tadalafil side effects are facial flushing (reddening), headaches, stomach upset, diarrhea, flu-like symptoms and nausea.

Tadalafil also may cause chest pain, low blood pressure, blurred vision and changes in color vision, abnormal ejaculation and priapism (prolonged and painful erection).