Viagra.
Viagra prevent the breakdown of nitric oxide, a chemical messenger that promotes relaxation and opening of the blood vessels that supply erectile tissue in the penis. Under the influence of nitric oxide, these vessels expand and stay dilated. Increased blood flow makes erectile tissue swell and compress the veins that carry blood out of the penis, resulting in a full erection.
PDE-5 inhibitors do not automatically trigger erections. Sexual stimulation also is needed to start the whole process. The medications enable a more complete response to sexual stimulation.
 Many clinical trials have shown that PDE-5 inhibitors improve erectile function regardless of the underlying cause or causes. Viagra  increase the number and quality of erections and sexual experiences in men with erectile dysfunction due to arteriosclerosis, diabetes, spinal cord injury, depression or the aftereffects of prostate cancer surgery.Â
The drugs have slightly different chemical compositions that affect how quickly they work and wear off. Other distinctions — for example, which drug may be best for men of different ages or with different medical conditions — aren’t known. No study has directly compared these three medications. Several factors may affect your choice of medication, including how well your body responds to one drug over another and how long you want the effects of each dose to last. Talk to your doctor about your options and personal preferences to help decide which one of these medications might work for you.
 Although these medications can help many people, not all men can safely take them. PDE-5 inhibitors may worsen certain medical conditions and interact with a number of drugs. Erectile dysfunction medications are dangerous when used with nitrate medications, such as nitroglycerin (Nitro-Dur, others), often prescribed to prevent or treat acute angina (chest pain due to coronary artery disease). Both types of medication dilate blood vessels, and their combined effects can cause dizziness, low blood pressure and loss of consciousness. In rare cases, men using PDE-5 inhibitors have suffered permanent, total vision loss due to nonarteritic anterior ischemic optic neuropathy (NAION). Because NAION and erectile dysfunction share many of the same risk factors, it’s unclear whether the drug or an underlying condition is responsible. If you’re considering an erectile dysfunction drug but have a retinal disorder, such as diabetic retinopathy, see your eye doctor first.