The Food and Drug Administration (FDA) has approved revisions to labels for anti-impotence drugs for a condition known as ‘transient global amnesia’, which is a sudden, temporary memory loss that can last a few hours.

The changes will be made on Pfizer´s Viagra and Eli Lilly & Co´s Cialis. Last May a similar revision was made on Bayer AG´s Levitra - co-marketed in the U.S. by GlaxoSmithKline PLC/Schering-Plough Corp.

The drug makers say the number of cases has been limited, but label changes usually mean that multiple users have reported health problems after taking a drug, even though the reports don´t necessarily mean that the drug caused the problem.

It is actually not clear whether the erectile dysfunction (ED) drugs are causing the memory loss, or if other factors are to blame. Transient global amnesia can occur in people who haven´t taken ED drugs, such as sudden immersion in cold or hot water or strenuous physical activity, and the underlying cause is unknown, according to the Mayo Clinic's Website. According to statistics, the incidence of this condition is approximately 2.9 cases per 100,000 patients.

New York-based Pfizer said there´s no evidence that Viagra causes the condition. An Eli Lilly spokeswoman said transient global amnesia was rare in Cialis users.

Some doctors, however, have suggested there may be a link between the drugs and amnesia. The memory loss episodes typically last about six hours.

Paradoxically, if this condition occurs after a sexual encounter in which the man used the drug, he may not remember having engaged in sex, or even who he is.

German neurologists who wrote an article in 2005 describing the case (such as above) of a 46-year-old man who had taken Eli Lilly´s Cialis, said too there had been 35 reports of amnesia in Viagra users between 1998 and 2001. They suggested there may be a causal link between the drugs and transient global amnesia, though they couldn´t rule out that other factors may have caused the condition.

Pfizer's Viagra leads the market with sales of $ 923 million for the first six months of 2008. Eli Lilly's Cialis sales were $ 699 million for the first half, while Levitra´s earnings were some $ 300 million dollars for the same period.