To understand how viagra works. First, you need to understand the process of how a man gets an erection. When a man gets sexually aroused the nervous system in the erectile tissue of your penis releases nitric oxide (NO) 

Man getting erections may seem pretty basic but they’re more complicated. Getting an erection involves annexation between your heart, blood vessels, hormones, nerves, and even your mood. 

During a man getting a get an erection cGMP (cyclic guanosine monophosphate) causes tissues in the penis to relax. the result of that blood flows into the penis and is trapped—up to eight times more blood than when the penis is flaccid. After some time this trapped blood gets released, the erection subsides, and blood flow returns to normal.

PDE-5(Phosphodiesterase type 5) is an enzyme that breaks down a molecule called cGMP, resulting in vasoconstriction or the narrowing of blood vessels.

Some time high levels of PDE-5 are found in the penis, lungs, and retina. PDE-5 is also found throughout the body within the smooth muscle cells of blood vessels and muscles. 

Viagra works for men by blocking PDE-5. When PDE-5 is blocked, cGMP does not get broken down. As a result of that, the tissues in the penis stay relaxed and engorged with blood.

If a man does not turn on, man’s brain will not stimulate the release of any nitric oxide and a man will not produce any cGMP.

ED (Erectile dysfunction) is when a man experiences issues getting an erection. Or keeping it long enough for sex. It’s also known as ED or impotence. It happens when insufficient blood streams to the penis, preventing an erection.

It’s been proven to help men with all degrees of ED. but remember one thing takes only when you need it.

Viagra is approved to treat ED erectile dysfunction (impotence) in men. Another 

The company of sildenafil (Revatio) is approved to treat high blood pressure associated with pulmonary arterial hypertension (PAH).

Upside 

• Viagra is approved to treat ED also known as impotence, in men. It increases blood flow to the penis following sexual stimulation and allows men to keep an erection hard enough for sex.
• Viagra only works following sexually aroused It works within 30 to 60 minutes but may be taken up to four hours before sexual activity.

Downside

 If your age is between 18 to 60  take no other medication or have no other medical conditions, side effects you are more likely to experience include

• Dyspepsia, headache, flushing indigestion, muscle or back pain, and visual disturbances
• Other side effects like a drop in blood pressure, nasal congestion insomnia or a runny nose and nosebleeds or Sudden hearing loss have also been reported. Some of the guys taking Viagra have had bleeding events; however, it has not been established if this is actually related to Viagra use.
• In some cases may decrease blood flow to the optic nerve causing sudden vision loss. The risk is bigger in people aged older than 50, or with diabetes, heart disease, high blood pressure, high cholesterol, who smoke, or with certain pre-existing eye problems
• In some cases, it may cause other side effects or lead to painful erections lasting longer than four hours. If this happens See your doctor immediately.
• It  may not be suitable for some people, including those with angina, chest pain, or other heart conditions, liver or kidney problems, blood or bleeding disorders, stomach ulcers, certain eye conditions, or a physical deformity of the penis (such as Peyronie’s disease)
• It may interact with some medications including certain antibiotics and antifungals, herbal remedies, HIV medicines and drugs used in the treatment of high blood pressure or prostate disorders
• Viagra gives no protection against sexually transmitted diseases 
• Viagra is available in general under the name sildenafil

Viagra is used to treat (ED) erectile dysfunction but will not have any effect in the absence of sexual stimulation. Stomach upsets, headache, and flushing are the main side effects.