Interactions between Viagra and other drugs


Uncategorized / Tuesday, December 12th, 2017

How Viagra can interact with other drugs

Viagra, along with the other prescription drugs for the treatment of erectile dysfunction, Cialis and Levitra, all belong to the class of drugs known as phosphodiesterase-5 inhibitors (PDE5 inhibitors). These drugs tend to be used by older men, as it is older men who are most likely to require treatment for erectile dysfunction.

However, older men may be taking other medications concurrently, to treat other conditions. It is here that the prescribing doctor or GP needs to be aware of any possible drug interactions that can occur, so that he can advise the patient accordingly.

How can Viagra and other erectile dyfunction drugs interact with other medications?

PDE5 inhibitors cause dilation of the blood vessels (to allow blood to enter the penis and produce an erection). They are known as vasodilators. They are also metabolised by an enzyme called CYP3A4 in the liver and intestines. Therefore, there are two main ways that drug interactions can occur:precription of Viagra in conjuction with another vasodilatorprescription of Viagra in conjunction with a drug that increases or decreases CYP3A4 activity.

Interactions with Vasodilators

The most common group at risk is men who take nitrates. This is a well known interaction and men taking nitrates should not be prescribed Viagra as dangerously low blood pressure can occur, possibly leading to heart attack, stroke and maybe death. Another class of drug, called alpha blockers, are used to treat conditions such as high blood pressure and hypertrophy of the prostate gland. There are certain precautions / dosage adjustments / timing of taking the drug that can be recommended with alpha blockers so as not to cause dangerous interactions. This is of particular interest for men who are being prescribed alpha blockers to treat an enlarged prostate gland, as they are more likely to be requiring Viagra.

Interactions with drugs that alter CYP3A4 activity

 

Drugs that increase CYP3A4 activity

These drugs tend to reduce the effectiveness of Viagra. Whilst this is not dangerous, it may stop the Viagra working. For example, the antibiotic rifampin causes a marked increase in CYP3A4 activity that can render even the highest dose of Viagra ineffective.

Drugs that decrease CYP3A4 activity

These drugs can reduce the normal metabolising of Viagra, leading to increased concentrations of the active ingredient in the blood, which can be toxic. If you are taking a drug that markedly decreases CYP3A4 activity, such as ketoconazole, itraconazole or ritonavir, your doctor may recommend a reduced dose of Viagra. You should also be aware that grapefruit juice decreases CYP3A4 activity, so do not drink grapefruit juice if you are taking Viagra.

You can see that it is important that you inform your doctor of any and all drugs you are taking, so that he can decide if it is safe to prescribe Viagra for you, or make any necessary adjustments. You should also check interactions with your doctor before starting any new drugs.