US scientists have claimed that one of the most commonly used selective serotonin re-uptake inhibitors, GlaxoSmithKline's Seroxat/Paxil, causes serious DNA damage in sperm cells, according to the report in New Scientist.

Dr Peter Schlegel and colleagues at Cornell Medical Center in New York City studied the effects of Paxil (paroxetine) on the sperm quality of 35 volunteers before and after five weeks of treatment. At first glance the men's sperm seemed normal in terms of volume and motility after the treatment. But closer inspection revealed that the number of sperm cells with fragmented or damaged DNA appeared to have risen sharply. On average, the proportion of sperm cells with damaged DNA rose from 13.8% before taking paroxetine to 30.3% after just four weeks.

The team will present the findings in November at a meeting of the American Society for Reproductive Medicine in San Francisco.

DNA fragmentation has been linked to serious fertility problems. In couples undergoing IVF, studies have found that where the man has more sperm with damaged DNA, fewer embryos form and those do form are less likely to implant successfully into the woman's uterus.

Douglas Carrell, an expert in male infertility at the University of Utah in Salt Lake City noted that as results of these problems, fertility specialists regarded a fraction of 30% cent of sperm with DNA damage as being "clinically significant". He added that the findings did not come as a surprise: "I think a lot of us around the world have had data that have pointed in this direction and have been suspicious," he said.

Janet Morgan, a spokeswoman for the UK-based drugs giant, said: "This study was not conducted by GSK, and therefore we are currently reviewing the investigators' findings. We take seriously our responsibility to ensure our medicines are used safely."

SSRIs are known to slow the movement of sperm through the male reproductive system, an effect that has been exploited to help treat premature ejaculation. Dr Schlegel speculated that this extra time spent travelling from the testes causes sperm to accumulate DNA damage.

Other experts, however, have pointed to shortcomings in the research. Dr Allan Pacey, a senior lecturer in andrology at the University of Sheffield, said: "It is a shame that the authors appear not to have conducted a randomised controlled trial which would be the most scientific way to investigate the drug's effects."

Dr Pacey added, however: "I agree that the results are of concern and need to be investigated further. The apparent increase in sperm DNA damage is alarming, although the level at which we think the damage becomes clinically significant is controversial to many scientists."