THERE is now a very remote chance that the Victorian threat to schoolboys that sex could blind them could be true.

In June 2002 the Australian Adverse Drug Reactions Advisory Committee issued a report on 773 cases reported to them over a three-year period of adverse effects of Viagra, with 65 of them concerning changes in vision. The greatest number of these were of a transitory change in the hue of everything looked at; the result of an increased blood supply to the retina which causes the visual field to take on a bluish tinge. There was also, very often, some sensitivity to light.

More important changes in vision after taking Viagra have been noted by other observers. These have included a temporary loss of vision and, very rarely, permanent blindness. A study at the University of Minnesota this year reported that a very small number of men who had taken Viagra had suffered an interruption of the blood flow to the optic nerve, a condition known as nonarteritic ischaemic optic neuropathy; in everyday language, a stroke in the optic nerve, which is the cranial nerve that leads to the retina.

It has always been realised that if someone takes a drug that reaches parts that other drugs cannot reach it would be likely that this effect would not be restricted to the penile arteries. Viagra causes vasodilation, hence its action in correcting erectile dysfunction.

Because Viagra causes dilation of peripheral blood vessels it may also produce headache, flushing, dizziness and nasal congestion. There is a close association between pelvic blood supply and nasal blood supply.

The vasodilation may bring about an immediate reduction in blood pressure and has been known to precipitate heart attacks, chest pain or strokes. It could not be said with any certainty that this actually caused them, as the men taking Viagra were often elderly and had pre-existing disease.

Millions of men take Viagra without harm but their doctors are careful to make certain that they are not taking other drugs that may also have a similar vasodilatory effect and patients are specifically asked about any heart conditions. A recent coronary or stroke makes the use of Viagra inadvisable.

Just as doctors prescribing Viagra check the cardiovascular system, so do they refuse to prescribe Viagra if the patient is known to suffer from retinal diseases. Patients are always closely questioned about any eye disease.

Viagra probably has a less-targeted influence on peripheral blood vessels than Levitra, a similar but more recent drug, and one that has a more specific action on the pelvic arteries. The risk from Viagra, although tiny, is real. All drugs have the potential to cause adverse effects but most patients consider the risks involved with the Viagra group of drugs as acceptable.

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