Best-selling antidepressant drugs may be making some men infertile, according to research reported yesterday. The warning follows a study of two men at Cornell Medical Centre in New York whose sperm counts dropped dramatically to almost zero while taking the drugs, but recovered to healthy levels whenever their medication was suspended.

The men were tested over a two-year period while being treated with the antidepressants Citalopram (Cipramil) or Sertraline (Lustral), which belong to the same class of drugs as Prozac and Seroxat, Britain's biggest-selling antidepressants. A further 12 men at the clinic have since been discovered to experience a similar but less dramatic fall in sperm counts while on the medication.

Doctors yesterday urged men receiving the drugs to combat depression not to stop taking them before consulting their GPs. While only two men at the clinic had a dramatic reaction to the drugs, the researchers believe large numbers of men may be affected to a lesser extent. The team, lead by Peter Schlegel, has launched an urgent clinical trial of 30 healthy men, to confirm how common the possible side-effect might be.

"These were men with normal sperm counts that went to nearly zero when they were on these antidepressants but returned to normal when they were off them. It's a dramatic effect and it's never been described before," said Professor Schelgel. "We believe that while it's had a profound effect on these two men, it could be having a significant but more subtle effect on many more men."

The previously unknown effect could be having a significant effect on couples trying to start families, Prof Schegel said.

Demand for antidepressants has soared in recent years, with doctors writing 19m prescriptions for Seroxat alone in 2004.