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"Our data suggests that cannabis users might be advised to stop using the drug if they are planning to try and start a family," study author Dr. Allan Pacey, a senior lecturer at the University of Sheffield in England, said in a university news release.
"Our knowledge of factors that influence sperm size and shape is very limited, yet faced with a diagnosis of poor sperm [size and shape], many men are concerned to try and identify any factors in their lifestyle that could be causing this," said Pacey.
The study, published June 5 in the journal Human Reproduction, doesn't prove that pot use impairs male fertility; it only suggests an association between the two.
The research included nearly 2,000 men who visited one of 14 fertility clinics and answered questions about their medical history and their lifestyle.
Of that group of men, 318 had sperm that was considered "abnormal." Less than 4 percent of their sperm was the correct size and shape, the researchers said. The information collected on these men was compared to data from the remaining 1,652 men who had sperm that was considered "normal."
The researchers examined how certain lifestyle choices affected the size and shape of the men's sperm and found "very few identifiable risks," Pacey said.
Although cigarettes and alcohol had little effect, the study revealed that sperm size and shape was negatively affected if the men were younger than age 30 and had smoked marijuana within three months.
Sperm samples produced during the summer were also nearly twice as likely to be considered "abnormal," the study showed.
The study authors noted that sperm size and shape may be linked to other factors that were not measured, such as the quality of a man's DNA.
Previous research has suggested sperm need to be of good size and shape to work their way through a woman's body after sex and fertilize an egg.
Men exposed to paint strippers and lead are also at risk of having sperm with poor shape and size, Nicola Cherry, an epidemiologist and professor formerly of the University of Manchester in England, said in the news release.
-- Mary Elizabeth Dallas
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SOURCE: University of Sheffield, news release, June 4, 2014