Be wary of any product that claims to be a natural form of Viagra. Herbal supplements aren't held to the same standards as prescription and over-the-counter medications — so it can be difficult to know which ones are safe or effective.

Prescription medications used to treat erectile dysfunction — sildenafil (Viagra), tadalafil (Cialis), vardenafil (Levitra), and others — work by increasing blood flow to the penis when you become sexually aroused. Some "herbal viagra" supplements are advertised as a cheaper alternative, available without a prescription.

Some of these products do contain substances that improve blood flow by relaxing the walls of blood vessels. However, none of them focuses specifically on blood vessels in the penis, as do Viagra and similar prescription drugs. As a result, these "herbal viagra" supplements can cause dangerously low blood pressure in some men — particularly men who take nitrate medications for heart problems.

In addition, some products marketed as "herbal viagra" have been pulled off the market because they contain undeclared prescription medications. It can be dangerous to take supplements that contain unknown amounts of prescription medications or other unidentified ingredients.

If you have erectile dysfunction, see your doctor to make sure it isn't caused by an underlying health problem or isn't a side effect of medications you take. Your safest bet in treating erectile problems is to rely on your doctor's evaluation and recommendations. Before using any herbal remedy or supplement, check with your doctor to make sure it's safe for you.