A case report on two patients taking the most common class of antidepressants, which includes the market leaders Seroxat and Prozac, has revealed a possible adverse effect on both the concentration and swimming ability of sperm.

Doctors at the Cornell Medical Centre, in New York, who were treating the two men for infertility, found that when the patients stopped taking selective serotonin reuptake inhibitors (SSRIs), their fertility problems disappeared, only to resume when they restarted antidepressants.

The first patient was taking citalopram, known as Cipramil in Britain. The second was on sertraline, which is sold as Lustral in Britain. The second patient then switched to a different SSRI, venlafaxine or Effexor. Again, his sperm count dipped, only to climb again when he came off the drug.

The implications of the study are limited by the very small number of patients, but similar effects have been reported in a another dozen patients. A clinical trial has begun of 30 men taking sertraline.

Peter Schlegel, who presented the research yesterday at the American Society for Reproductive Medicine conference, in New Orleans, said: “The patients had normal sperm counts and motility before medication. On the medication they have severe deterioration of both. The same patients going on and off medication had the same pattern. It shows a strong association.”

Impotence and delayed ejaculation are common side-effects of the drugs, and Dr Schlegel believes that the drugs may be preventing sperm from getting into semen.

Allan Pacey, senior lecturer in andrology at the University of Sheffield, said: “There does seem to be a major correlation. Maybe this is an unknown side effect of these drugs that is only just coming to light.”

The researchers warned patients taking SSRIs not to abandon their use of the drugs over concerns for their sperm count, as sudden changes in treatment may worsen psychiatric conditions.

SSRIs are the most commonly prescribed antidepressants in Britain, and are taken by between two and three million people.

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