Bad news for marijuana-using men: Researchers from the Universities of Sheffield and Manchester have found that smoking marijuana is among the few lifestyle factors that can reduce fertility by affecting the size and shape of sperm. There is, however, good news: Other factors, like drinking alcohol, smoking tobacco, being obese, and wearing tight underwear appeared to have no effect on sperm morphology.

“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. 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 lead author Dr Allan Pacey, Senior Lecturer in Andrology at the University of Sheffield.

The study, which recruited 2,249 men from 14 fertility clinics around the UK, asked participants to fill out very detailed surveys concerning their lifestyle and medical history. 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 4% was the correct size and shape and a control group of 1,652 men where this was above 4% and therefore considered “normal” by current medical definitions.

Poor sperm morphology is a problem: Earlier studies found that only normally sized/shaped sperm are able to make their way to eggs within a woman’s body, and abnormally shaped sperm don’t swim well due to their less-efficient shapes.

The takeaway from the study is that for men with abnormal sperm morphology need not delay fertility treatment in favor of attempts to change lifestyle factors (unless that factor is weed). It’s worth noting, though, that only measuring morphology ignores several other sperm characteristics, such as the quality of the DNA contained within. It’s unclear whether things like smoking, drinking, etc. have any effect in those areas.