Use of cannabis by young men can be a risk to their fertility affecting the size and shape of their sperm, say researchers.
In the world's largest study to investigate how common lifestyle factors influence the size and shape of sperm (referred to as sperm morphology), a research team from the Universities of Sheffield and Manchester also found that sperm size and shape was worse in samples ejaculated in the summer months but was better in men who had abstained from sexual activity for more than six days.
For the study, scientists recruited 2,249 men from 14 fertility clinics around the UK1 and asked them to fill out detailed questionnaires about their medical history and their lifestyle.
Men who produced ejaculates with less than four percent normal sperm were nearly twice as likely to have produced a sample in the summer months (June to August), or if they were younger than 30 years old, to have used cannabis in the three month period prior to ejaculation.
The study has been published in the medical journal Human Reproduction.