Drugs taken by millions of men to alleviate depression may affect their fertility, say US scientists.
A small number of healthy men given the antidepressant paroxetine for four weeks had far higher levels of sperm with damaged DNA.
The results, reported in New Scientist magazine, do not necessarily mean these men would have serious problems becoming a father.
However, a UK fertility specialist said they were a "cause for concern".
Paroxetine, sold as Seroxat or Paxil, is one of the most commonly prescribed antidepressants in the UK.
This is the second study by a team of researchers at Cornell Medical Center in New York which points to a possible effect on sperm quality.
They recruited 35 healthy volunteers who provided sperm samples before and during paroxetine treatment.
Under the microscope, there appeared to be not much difference between the "before" and "after" samples, with the shape and movement of sperm apparently normal in both samples.
However, tests on "DNA fragmentation" produced a different result.
Some sperm with DNA problems can be found in every sample, and 13.8% of sperm cells in those produced before treatment were found to be fragmented.
However, at the four week mark, this had risen to 30.3%.
A key question is whether this change would be enough to affect overall fertility, or whether the remaining 70% of unaffected sperm would be enough to produce a viable pregnancy.
In couples undergoing IVF, studies have found that couples where the man's sperm has higher levels of DNA damage produce fewer embryos, and their embryos are less likely to implant successfully in the womb.
More work needed
Dr Allan Pacey, senior lecturer in Andrology at the University of Sheffield, said that while there had been "sporadic reports" that antidepressants could affect semen quality, more research would be needed to help scientists evaluate the risk.
"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.
"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 drugs effects, but I agree that the results are of concern and need to be investigated further."
The drug's maker, GlaxoSmithKline said it intended to review the study's findings.
Marjorie Wallace, from the mental health charity Sane, said that patients should wait for larger studies: "While these results may be worrying for people taking antidepressants who hope to conceive, it is important to note that this is a preliminary study with a small sample group.
"We would be worried if this study caused patients to stop their treatment and would urge anyone with concerns to consult their doctor.
"Antidepressants can be a lifeline for many people, and the risk of relapse must be borne in mind in balancing the risks and benefits of these drugs."
Dr Andrew McCulloch, of the Mental Health Foundation, said: "Most medications carry some level of risk, and antidepressants are no different.
"They are powerful drugs, so in a sense it is no surprise that research is discovering more about their impact on the body.
"More investment is needed in other mental health treatments such as talking therapies and exercise therapy.
"However, we should remember that SSRIs have changed many lives for the better and that any decision about changing or stopping your medication should be discussed with your doctor."