Viagra is a powerful drug that increases blood flow to the penis so you can get and maintain an erection. It’s effective, but it can also cause some side effects.

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Illustration by Anthony Agreste

Viagra is a brand-name version of the generic drug sildenafil. It’s a phosphodiesterase type 5 (PDE5) inhibitor. PDE5 is an enzyme that regulates certain chemicals in your blood, but as a result can make it harder for you to get and keep an erection.

Viagra is used to treat erectile dysfunction (ED). While it helps you temporarily maintain an erection so you can have sex, it doesn’t cure ED. It doesn’t affect sexual desire either. You still need mental or physical stimulation to get an erection.

This medication is only available with a doctor’s prescription.

It takes a finely choreographed series of events to produce an erection. It begins with arousal signals from your brain, and it all hinges on good blood flow to the penis.

Within the penis are two chambers called the corpora cavernosa. Nitric oxide (NO) is released in the chambers during sexual stimulation. NO activates an enzyme called guanylate cyclase. That increases levels of cyclic guanosine monophosphate (cGMP), which causes muscles to relax.

The chambers also contain a network of blood vessels. When those blood vessels relax and widen, blood rushes in. The resulting pressure is what causes an erection.

PDE5 can dampen the effect of cGMP. Viagra works by inhibiting PDE5.

Viagra is rapidly absorbed into the bloodstream. Maximum concentrations are reached within about an hour.

One of the more common side effects is flushing, or redness.

This medication can also cause a decrease in blood pressure, particularly one to two hours after taking it. If you already have low blood pressure, discuss the pros and cons of Viagra with your doctor.

For most people, sexual activity is good for cardiovascular health. However, if you have cardiovascular disease, you should ask your doctor if it’s safe for you to take Viagra. You should also avoid Viagra if your doctor advised you not to have sex.

You shouldn’t take Viagra if you’ve had a stroke or heart attack or if you have unstable angina.

Certain drug interactions can harm your heart. Avoid taking PDE5 inhibitors if you also use long-lasting alpha-blockers or take medications that contain nitrates.

Viagra can be quite effective, but it’s no magic pill. It does nothing for the libido. You still need some kind of stimulation to get an erection.

The effects of Viagra usually last about 4 hours, though they may last longer for some men. A rare, but serious side effect, is priapism. That’s when you get an erection that lasts for a long period of time. It can become quite painful.

If you have an erection that lasts for more than 4 hours, seek immediate medical attention.

You should also be wary of PDE5 inhibitors if you have an anatomical abnormality of the penis. If you have Peyronie’s disease, your doctor may advise against taking Viagra.

Viagra is a temporary fix and doesn’t cure ED. It offers no protection from sexually transmitted infections (STIs).

Viagra helps to improve blood flow to the penis, but your brain is still your most valuable sex organ. Viagra won’t work if you’re not in the mood.

Some potential side effects of Viagra are headache and a runny or blocked nose, or a nosebleed. Some men feel lightheaded or dizzy. In rare cases, Viagra can cause fainting. Some men taking PDE5 inhibitors report back or muscle pain.

It’s not common, but some men experience ringing in the ears, hearing loss, or vision loss after taking PDE5 inhibitors.

Avoid PDE5 inhibitors if you have a history of an eye condition called non-arteritic anterior ischemic optic neuropathy (NAION). NAION is characterized by the interruption of blood flow to the optic nerve.

If you have hearing or vision loss while taking Viagra, seek immediate medical help.

Viagra is dispensed in a film-coated tablet. You can take Viagra with or without food, and it’s best to take it about an hour before you plan to have sex.

Don’t take Viagra more than once per day.

About 80 percent of Viagra leaves your body in your feces. The rest is washed out with your urine.

One fairly common side effect of Viagra is indigestion or stomach upset. PDE5 inhibitors can sometimes cause nausea or vomiting.