Viagra, which belongs to the class of drugs called phosphodiesterase-5 inhibitors (PDE5i), are widely used, especially in older men

Because older men are more likely to be concurrently taking other medications, they should be aware of the potential for dangerous interactions when ED drugs are mixed with other prescription drugs like Viagra.

Genuine Viagra are drugs that cause the dilation of blood vessels (vasodilators) and this effect can be magnified when they are used with other medications with similar vasodilating effects.

It is this vasodilating effect of Viagra that increases blood flow to the penis causing an erection.

Viagra interactions, however, result from the fact that these drugs are metabolized by the enzyme CYP3A4 in the intestine and liver.

CYP3A4 is the most important enzyme for drug metabolism, and many medications can interact with ED drugs such as Viagra by decreasing or increasing CYP3A4 activity, causing ED drug levels to increase or decrease, respectively.

People who take nitrates (drugs commonly used to dilate the blood vessels in the heart for patients with coronary artery disease, such as nitroglycerin), either on a regular schedule or as needed, should not use Viagra.

A dangerous reduction in blood pressure may occur, potentially leading to a heart attack or stroke, and so Viagra should not be used in combination.

ED drugs, such as Viagra also can enhance the blood pressure-lowering effects of alpha blockers such as doxazosin (CARDURA), prazosin (MINIPRESS), terazosin (HYTRIN), tamsulosin (FLOMAX) and alfuzosin (UROXATRAL).

These drugs are used to treat both high blood pressure and enlarge prostate glands. If you are on an alpha blocker, consult your physician before taking an ED drug such as Viagra; he or she may recommend dosage adjustments or other precautions.


Drugs that inhibit CYP3A4 can substantially increase plasma concentrations of ED drugs such as Viagra, potentially leading to toxicity.

If you are taking any of these drugs, check with your physician before taking an ED drug such as Viagra.

Your physician may advise a dosage reduction of the ED drug such as Viagra especially if you are taking a potent CYP3A4 inhibitor such as ketoconazole, itraconazole or ritonavir. Keep in mind that grapefruit juice is also a CYP3A4 inhibitor.


Drugs that increase CYP3A4 tend to reduce the effect of ED drugs such as Viagra.

This is not dangerous, of course, but it may render the ED drug such as Viagra, ineffective.

If the increased CYP3A4 activity is marked — such as in patients taking the antibiotic rifampin, even the highest recommended dose of the ED drug such as Viagra may be ineffective.

Again, if you are taking any medications, talk to your physician before starting an ED drug such as Viagra.


Many medications have the potential to interact with ED drugs such as Viagra, so it is important to make sure that the physician prescribing the ED drug is aware of all the other medications you are taking. If you start or stop other medications after receiving a prescription for an ED drug, check to make sure your ED drug effect will not increase or decrease as a result.

For more news stories about Viagra please click here.