Cannabis use may put young men's fertility at risk, a new study suggests.

According to the findings, the drug appears to affect the size and shape of sperm in men under the age of 30.

UK scientists set out to assess how common lifestyle factors influence the size and shape of sperm - this is known as sperm morphology. Studies suggest that sperm with poor morphology are less efficient because they cannot swim as well as a result of their abnormal shape.

The scientists looked at over 2,200 men attending 14 fertility clinics around the UK. From these, they focused on 318 men who produced sperm of which less than 4% was the correct size and shape. These were compared to over 1,600 men who were considered to have ‘normal' sperm.

The study found that men under the age of 30 were almost twice as likely to have sperm that was less than 4% normal if they had used cannabis in the three months beforehand.

The study also found that overall, sperm size and shape was worse if ejaculated during the summer months. Drinking alcohol and smoking appeared to have little effect.

"Our knowledge of factors that influence sperm size and shape is very limited, yet faced with a diagnosis of poor sperm morphology, many men are concerned to try and identify any factors in their lifestyle that could be causing this.

"It is therefore reassuring to find that there are very few identifiable risks, although our data suggests that cannabis users might be advised to stop using the drug if they are planning to try and start a family," commented the study's lead author, Dr Allan Pacey, of the University of Sheffield.

Details of these findings are published in the journal, Human Reproduction.