Half the men had optic nerve damage, diagnosed as non-arteritic anterior ischaemic optic neuropathy (NAION), the rest were free of this.

All the patients in the study were asked about their lifestyle, including whether they smoked and drank alcohol, whether they had been diagnosed with heart disease, diabetes, or high blood pressure and prescribed treatment for these conditions and if they had been prescribed Viagra and Cialis.

Men with optic nerve damage were no more likely to have taken the impotency drugs than those in the comparison group but men who had had a heart attack were 10 times more likely to have optic nerve damage if they had taken Viagra or Cialis before being diagnosed with the condition.

Men with high blood pressure were also more likely to have optic nerve damage if they had taken these drugs, although this was not statistically significant.

The authors of the research warn that their study is small, but suggest that the drugs may reduce the blood flow to the anterior optic nerve, which can cause tissue damage.

They urge any patient who has endured a sudden severe loss of sight, and has been prescribed Viagra or Cialis, to contact a doctor.

However, Pfizer has said there were no reports of NAION in the trials of the drug carried out before it was licensed and Lilly ICOS, which makes Cialis, said there had been a small number of cases of NAION in people taking the drug but this information was now included in the prescribing information.

In a statement, Lilly said it was difficult to establish a firm link as most patients taking impotency drugs had other risk factors, which made them potentially vulnerable.