Viagra is used in the treatment of men with erectile dysfunction. Viagra blocks the enzyme known as phosphodiesterase-5 (PDE-5), which leads to smooth muscle relaxation and an increase in blood flow to the penis, and thus to an erection.

Clinical trials have shown that Viagra is well tolerated but, like all medicines, it may cause side effects. One effect that has attracted particular attention is visual disturbance, sometimes referred to as ‘blue vision’. It happens more frequently when high doses of Viagra are taken or in cases of drug abuse. It may occur because Viagra also blocks PDE-6 (although to a much lesser degree than PDE-5), which is involved in converting light into electrical signals in the light-sensitive cells of the eye.

Changes in blue/green colour discrimination were found in some men given single Viagra doses of 100 mg (the maximum recommended dose). Effects were mild and temporary, seen at one hour after dosing and disappearing after two hours. Viagra did not affect visual acuity (clearness) or contrast sensitivity in this study. Another study demonstrated that Viagra had no clinically relevant effects on the vision of patients with early age-related macular degeneration (an eye disorder resulting in loss of central vision).

In clinical trials where Viagra was taken when needed and at the recommended dose, 3% of men reported abnormal vision, which was mainly colour tinge to vision, increased sensitivity to light or blurred vision. The number of events increased with dose and with doses above the recommended dose range.

Men should be advised to stop taking Viagra and notify a doctor if they experience visual defects. They should also avoid driving as it may reduce their concentration. These side effects are usually mild to moderate in severity and temporary, disappearing as the effects of the drug wear off.

Rarely, in post-marketing studies, non-arteritic anterior ischemic optic neuropathy or NAION that causes reduced vision has also been associated with use of PDE5 inhibitors, including Viagra. However, it is not certain that these medicines caused NAION as these men also had other risk factors (such as diabetes, coronary artery disease, and high blood pressure) for this condition. As a precaution, Viagra is not recommended for men who have NAION and loss of vision in one eye. It is also not advised for use by men with a genetic degenerative condition of the retina (such as retinitis pigmentosa) because the effects of the drug in these people are unknown.

So where does this leave us? None of this indicates that taking Viagra is likely to seriously affect eye sight. However, as with all medications, it does indicate that it is preferable to seek advice before taking Viagra and to take the dose recommended by your doctor. It would appear that doctors are agreed that there is no risk here and some of the claims being made against Pfizer are opportunistic to say the least.