The effectiveness of Viagra, the world's most popular anti-impotency drug, may wear off after two years of use, according to details of a study carried in the latest edition of the British Medical Journal.
The base ingredients for the pill are manufactured at pharmaceutical company Pfizer's plant in Ringaskiddy, Co Cork.
A research project by the University of Alabama, Birmingham in the US has suggested that the drug sildenafil (Viagra), which combats erectile dysfunction, may stop working for patients after two years.
Dr Rizk El-Galley and colleagues from the university published their findings in the US Journal of Urology.
They interviewed 151 men who had taken the drug in 1997 and then followed their "progress" for three years.
In the initial stages of the research project, 74 per cent of the men reported that 25-100 mg of the drug enabled them to maintain erections, according to the researchers.
The investigators interviewed 82 of the men again in August last year, when 43 of them were still using the drug.
A total of 16 of the 43 men said they had needed to increase their dose by 50 mg to achieve an adequate erection. The men reported it had taken between one month and 18 months for the treatment to lose its effects.
However, Mr Geoff Cook, a New York-based spokesman for Pfizer, said the research project's findings were "contrary" to clinical studies carried out by his company.
"Fifteen million men have taken it worldwide. This drug works very well. In all the studies we've done we've never seen an effect like this," he said.
Mr Cook said the researchers had produced no evidence to call into question the efficacy of the drug.
He suggested that other medical conditions frequently experienced by men with erectile problems, such as diabetes and cardiovascular disease, could be to blame for the findings.
He said if men taking Viagra continued to have erectile problems they should contact their doctor immediately.
"Medical evidence to date shows this just doesn't happen and if it is happening it means there's some other problem."
An estimated 10 per cent of the Irish male population suffers from varying degrees of erectile failure.
