How Viagra Works
Viagra (also known as Sildenafil) is a prescription drug used to enhance erection during sexual stimulation. This is not the kind of drug to be used on one's own, since it is definitely not for everyone. The medicine is available in film-coated tablets based on sildenafil citrate, and they are only sold by prescription. Tablets contain 25, 50 or 100 mg of sildenafil citrate.
Sildenafil is a potent PDE5 inhibitor. Understanding the drug's mechanism of action requires a look at the mechanism of erection. During sexual excitement, nitric oxide is released into the penile chambers and stimulates the production of cyclic guanosine monophosphate (cGMP) responsible for muscular relaxation in the chambers. Sildenafil's action consists in blocking the photodiestrase type 5 that keeps the muscle in tension and therefore impedes blood flow to the chambers. Inhibition of PDE5 increases cGMP level, since PDE5 has a neutralizing effect on cGMP. Consequently, the less PDE5 there is in the chambers (corpus cavernosum), the more cGMP is there to loosen the smooth muscle and the more blood gets into the penis, which is crucial for stable erection.
Viagra's selective action is explained by the fact that it is scores and even hundreds of times more potent on PDE5 than on other enzymes, such as PDE1, PDE2, PDE6, etc. It is about 4,000 times more selective to PDE5 than to PDE3 - the enzyme that bears a direct relation to the functioning of the heart. However, Sildenafil is only about 10 times as selective to PDE5 as it is to PDE6. The latter is an enzyme responsible for retinal phototransduction. Therefore, Sildenafil may cause unexpected vision problems.
Viagra tablets are intended for oral administration. Dosage should be defined with the help of a qualified physician. In order to reduce the risk of side effects, it is recommended to start with small amounts of Tadanafil. On average, the drug's action becomes observable 30 minutes after administration, and hence it is best taken about 30 minutes before sex.
Sildenafil does not make away with the cause of impotence. Nitric oxide, which stimulates the synthesis of cGMT, is only produced during sexual excitement. Therefore, the drug is only efficacious when a man is in a state of excitement. The drug is absorbed very quickly, so it should be taken with food. Its effect lasts about 4 hours. Viagra should not be administered more than once each day. It should not be co-administered with other ED drugs, alpha-blockers or nitrates.