Although Viagra, Levitra, and Cialis all work by inhibition of PDE5, tadalafil's distinguishing pharmacologic feature is its longer half-life (17.5 hours) compared with Viagra and Levitra (4-5 hours). This longer half-life results in a longer duration of action and is, in part, responsible for the Cialis nickname of the "weekend pill." This longer half-life also is the basis of current investigation for tadalafil's use in pulmonary arterial hypertension as a once-daily therapy. At present, sildenafil (trade name Revatio) is approved in various regions worldwide as a 3-times daily therapy for pulmonary arterial hypertension.

Penile erection during sexual stimulation is caused by increased penile blood flow resulting from the relaxation of penile arteries and corpus cavernosal smooth muscle. This response is mediated by the release of nitric oxide (NO) from nerve terminals and endothelial cells, which stimulates the synthesis of cGMP in smooth muscle cells. Cyclic GMP causes smooth muscle relaxation and increased blood flow into the corpus cavernosum. The inhibition of phosphodiesterase type 5 (PDE5) enhances erectile function by increasing the amount of cGMP. Tadalafil (as well as sildenafil and vardenafil) inhibits PDE5. Because sexual stimulation is required to initiate the local release of nitric oxide, the inhibition of PDE5 by tadalafil has no effect in the absence of sexual stimulation. A 20 mg dose of tadalafil is comparable to a 100 mg dose of sildenafil (Viagra).However, the recommended starting dose of Cialis in most patients is 10 mg, taken prior to anticipated sexual activity. The dose may be increased to 20 mg or decreased to 5 mg, based on individual efficacy and tolerability

Tadalafil is currently undergoing clinical trials for the treatment of pulmonary arterial hypertension. In patients with pulmonary arterial hypertension, it is believed that there is an imablance of the PDE5/NO system in the pulmonary vasculature that favors selective vasoconstriction of the pulmonary artery. Investigation of tadalafil in this disease assumes that PDE5 inhibition will result in pulmonary artery vasodilation, thus lowering pulmonary artery pressure and pulmonary vascular resistance. These physiologic changes may then reduce the workload of the right ventricle of the heart. Right heart failure is the main consequence of pulmonary arterial hypertension.