Investigating how common lifestyle factors influence the size and shape of sperm, 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.
However, other common lifestyle factors reported by men, including smoking cigarettes or drinking alcohol, appeared to have little effect.
The study recruited 2,249 men from 14 fertility clinics around the UK and asked them to fill out detailed questionnaires about their medical history and their lifestyle.
Reliable data about sperm morphology was only available for 1,970 men and so the researchers compared the information collected for 318 men who produced sperm of which less than four per cent was the correct size and shape and a control group of 1,652 men where this was above four per cent and therefore considered 'normal' by current medical definitions.
Men who produced ejaculates with less than four per cent 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.
"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," lead author Dr Allan Pacey, Senior Lecturer in Andrology at the University of Sheffield, said.
"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," said Pacey.
Studies in the laboratory suggest that sperm with poor morphology swim less well because their abnormal shape makes them less efficient.
The study was published in the journal Human Reproduction.