CIALIS (tadalafil), an oral treatment for erectile dysfunction, is a selective inhibitor of cyclic guanosine monophosphate (cGMP)-specific phosphodiesterase type 5 (PDE5). Tadalafil has the empirical formula C22H19N3O4 representing a molecular weight of 389.41. The structural formula is: 
The chemical designation is pyrazino[1′,2′:1,6]pyrido[3,4-b]indole-1,4-dione, 6-(1,3-benzodioxol-5-yl)-2,3,6,7,12,12a-hexahydro-2-methyl-, (6R,12aR)-. It is a crystalline solid that is practically insoluble in water and very slightly soluble in ethanol. CIALIS is available as film-coated, almond-shaped tablets for oral administration. Each tablet contains 5, 10, or 20 mg of tadalafil and the following inactive ingredients: croscarmellose sodium, hydroxypropyl cellulose, hypromellose, iron oxide, lactose monohydrate, magnesium stearate, microcrystalline cellulose, sodium lauryl sulfate, talc, titanium dioxide, and triacetin. Mechanism of Action 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 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. Studies in vitro have demonstrated that tadalafil is a selective inhibitor of PDE5. PDE5 is found in corpus cavernosum smooth muscle, vascular and visceral smooth muscle, skeletal muscle, platelets, kidney, lung, cerebellum, and pancreas. In vitro studies have shown that the effect of tadalafil is more potent on PDE5 than on other phosphodiesterases. These studies have shown that tadalafil is >10,000-fold more potent for PDE5 than for PDE1, PDE2, PDE4, and PDE7 enzymes, which are found in the heart, brain, blood vessels, liver, leukocytes, skeletal muscle, and other organs. Tadalafil is >10,000-fold more potent for PDE5 than for PDE3, an enzyme found in the heart and blood vessels. Additionally, tadalafil is 700-fold more potent for PDE5 than for PDE6, which is found in the retina and is responsible for phototransduction. Tadalafil is >9,000-fold more potent for PDE5 than for PDE8, PDE9, and PDE10 and 14-fold more potent for PDE5 than for PDE11A1, an enzyme found in human skeletal muscle. Tadalafil inhibits human recombinant PDE11A1 activity at concentrations within the therapeutic range. The physiological role and clinical consequence of PDE11 inhibition in humans have not been defined. What is Cialis? Cialis (tadalafil) is in a class of medications known as PDE-5 inhibitors, which are used to treat cases of male impotence. Cialis is manufactured by Lilly ICOS LLC.
How does Cialis work? Cialis acts in the same way as Viagra, by blocking an enzyme called phosphodiesterase-5, or PDE-5. This helps the smooth muscles in the penis relax and widen, which allows for more blood to enter. As a result, you gain erections faster and easier. What's the difference between Viagra and Cialis? Cialis comes in smaller doses and produces fewer side effects. Cialis also works much faster than Viagra. In clinical trials, the majority of men who took the drug were able to engage in sexual intercourse within 30 minutes or less. The studies also indicated that Cialis stays in the system for up to 24 hours. That's 20 hours difference than the traditional Viagra treatment.
Test results have also shown that Cialis has no adverse side effects on vision or the heart. It's believed that Cialis is more selective than Viagra at targeting PDE5 receptor sites. Besides controlling blood flow to the penis, PDE5 enzymes play in essential role in the action of cells in the brain, heart, kidney, and eyes.
How well does Cialis work? Test results showed that out of 700 participants at least 88% of men experienced improvement with their erections. What's the normal dose? In clinical testing most men were administered one 10 or 20 mg tablets per day. Are there any side effects? The most common side effects reported in the study were headache, myalgia (muscle pain), dyspepsia (upset stomach following meals) and back pain. The severities of these side effects were minimized with lower doses.
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