Two medical experts say they believe Viagra is a generally safe drug, and they need to know much more before worrying it may increase one's risk of vision problems or even blindness.

The U.S. Food and Drug Administration is investigating reports that 43 people who took either Viagra (38) or a related drug (Cialis, 4; Levitra, 1) developed a type of blindness that is normally associated with heart disease or diabetes.

To put that number in perspective, an estimated 23 million men worldwide have used Viagra to treat erectile dysfunction since it came on the market in 1998.

"With the millions of users around the world, 43 men having blindness -- it doesn't mean you shouldn't pay attention to it, but it certainly doesn't establish cause and effect," said Dr. Jerald Bain, a University of Toronto professor and endocrinologist at Mount Sinai hospital.

Further research would have to be done into the affected mens' medical histories, he said.

"Were they diabetic? Were they prone to blindness for some other reason? ... Did they suddenly become blind an hour after taking a pill?" Bain told CTV.ca.

According to a BBC report, the vision problems occurred within 36 hours of taking Viagra.

Dr. Howard Pomeranz, the Minneapolis ophthalmologist who noticed the connection, told ABC News it was the timing that made him suspicious.

Pfizer responded with this: "There is no evidence showing that (vision loss) occurred more frequently in men taking Viagra than men of similar age and health who did not take Viagra."

Dr. Peter Pommerville, a B.C. urologist and vice chair of the Canadian Male Sexual Health Council, told CTV.ca that the type of blindness -- NAION, or non-arteritic anterior ischemic optic neuropathy; a swelling of the optic nerve -- is caused in part by blood vessel constriction related to hypertension, diabetes and high cholesterol.

All of the men had those risk factors.

"Viagra is a vaso-dilator," Pommerville said, meaning it opens blood vessels up, not constricts them.

He noted this particular story had its genesis in a brief note published earlier this year in the Journal of Neuro-ophthalmology.

Viagra has been around since March 1998 in the United States, and a year later in Canada.

The blue diamond-shaped pill started out as a potential heart drug, but researchers found it helped direct blood flow to another part of the male anatomy.

About five years later, some other related drugs came on the market -- Cialis and Levitra. All three belong to the class of drugs known as pde-5 inhibitors.

Known side effects

As with any drug, Viagra (more properly known as sildenafil citrate) has side effects.

Here is what you see in the fine print on the Viagra website produced by Pfizer, the drug's manufacturer:

• Those who take nitrate drugs for angina shouldn't use Viagra.
• Patients taking an alpha blocker for high blood pressure or prostate problems should talk to their physician. Viagra and an alpha blocker shouldn't be taken within four hours of each other.
• Pfizer recommends the lowest dose of 25 mg for those aged 65, men with severe liver or kidney problems or those who are on protease inhibitors.
• Those who have erections lasting more than four hours should consult with a doctor.
• Headache, facial flushing, and upset stomach are the most common side-effects.
• "Less common are bluish or blurred vision, or being sensitive to light. These may occur for a short time."

Pommerville noted that's because Viagra can increase blood flow to the eye, particularly the rods and cones in the retina. But that's unrelated to NIAON.

About a decade ago, Pommerville recalled there was a three-part study published in the prestigious Journal of the American Medical Association claiming men shouldn't have vasectomies because it would put them at increased risk of prostate cancer.

"It was crap," he said of the study. But for years afterward, men who wanted a vasectomy were worried doing so would mean getting prostate cancer.

Pommerville worried the same backlash could happen against Viagra, even though it's a generally safe drug that can help men who suffer from erectile dysfunction -- and he estimated there are three million of them in Canada.