Smoking cannabis can reduce a man’s fertility by altering the size and shape of his sperm, research at Sheffield University has shown.
More surprisingly, sex in the summer months has a similar effect, scientists found.
Conversely abstaining from sexual activity for more than six days improved the ‘morphology’ of sperm.
There was also reassuring news for prospective fathers who might be considering drastic lifestyle changes.
Common lifestyle factors including smoking cigarettes and drinking alcohol had little effect on sperm quality.
Lead scientist and senior lecturer Dr Allan Pacey said: “The take-home message is, if you’re a habitual cannabis user, stop, and you need to stop for at least three months.”