Study shows cannabis smoking affects male fertility

Men who smoke cannabis may be damaging their sperm and should give up the habit if they are trying to start a family, according to new research. It found no evidence of a similar effect from drinking alcohol or conventional smoking.

The study by the University of Sheffield into the impact of lifestyle on the size and shape of sperm also found that sperm quality is worse in summer months, but better in men who had abstained for more than six days.

Nearly 2,300 men attending 14 fertility clinics were recruited for the study, published in the medical journal, Human Reproduction, and asked to fill out detailed questionnaires about medical history and lifestyle.

Men who produced low-quality sperm, particularly if they were younger than 30, were twice as likely than those with higher-quality sperm to have used cannabis in the three months beforehand .

Previous research has shown that only high-quality sperm is able to make its way to the womb to fertilise, suggesting that lower-quality sperm “swim less well because their abnormal shape makes them less efficient”, the university said.

"Our knowledge of factors that influence sperm size and shape is very limited," said the head of the study, Dr Allan Pacey, senior lecturer in andrology at the university.

“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,” he said.

Despite the results, the jury is still out on the effects of alcohol, or smoking on sperm, since other qualities besides its shape – such as the quality of the DNA held within it – were not measured by the study.