Sperm from men who smoke cannabis regularly may fail to fertilise eggs because of premature burn-out, scientists said yesterday.

A study by scientists at Buffalo University school of medicine in New York showed the sperm move too fast, too early. Cannabis smokers also produced fewer sperm than non-drug takers.

The findings, presented at the annual meeting of the American Society of Reproductive Medicine in San Antonio, Texas, suggest that cannabis use can impair male fertility.

The scientists compared sperm from 22 cannabis smokers and 59 fertile men. Sperm were counted and tested for their ability to move.

The team also assessed "hyperactivity", a burst of vigorous swimming which is supposed to occur as sperm approach an egg.

Chief researcher Lani Burkman said: "The sperm from marijuana smokers were moving too fast, too early. The timing was all wrong. These sperm will experience burn-out before they reach the egg and would not be capable of fertilisation."

The amount of seminal fluid and number of sperm from cannabis smokers were also significantly reduced. The cannabis smokers were frequent users who had taken the drug an average of 14 times a week for about five years.

The study is the first to compare sperm from cannabis users with that from men with confirmed fertility by looking at specific swimming behaviour.

Dr Burkman said: "The active ingredients in marijuana are doing something to sperm."

It was unclear how cannabis upset the sperm's swimming programme. It was possible the active ingredient in cannabis known as THC stimulated sperm cells to disturb their timing. Alternatively, the drug might be affecting natural inhibitory mechanisms.

· A cup of strong coffee not only helps men feel awake, it makes their sperm swim more strongly, boosting fertility, according to researchers at the University of Sao Paolo in Brazil.

Tests on 750 men found regular coffee drinkers had more powerful sperm than those who did not touch caffeine.