According to a study by Harvard University, smoking cannabis occasionally may improve men's fertility by stimulating sperm production.

Published in the journal Human Reproduction, the results have taken the scientific community by surprise because previous research pointed to a negative effect for cannabis on male fertility.

The analysis of more than 662 men found that those who had never taken the drug were more than twice as likely to have sperm concentrations below a "normal" threshold, while the improved fertility in moderate users may be explained by a boost to the endocannabinoid system, known to play a role in sperm levels.

Dr Jorge Chavarro, from the Harvard T.H. Chan School of Public Health, said: "These unexpected findings highlight how little we know about the reproductive health effects of marijuana, and in fact of the health effects of marijuana in general.

"Our results need to be interpreted with caution and they highlight the need to further study the health effects of marijuana use."