Genetics and stem cells

Cannabis and sunshine may damage sperm quality

''Cannabis doubles younger men's risk of infertility, study finds,'' reports The Independent. The same study also found a drop in sperm quality during the summer months.

The paper reported on the results of a study that saw men attending fertility clinics, looking at the effects of lifestyle on one element of male fertility – known as sperm morphology, which refers to the size and shape of sperm.

Sperm with abnormal sperm morphology are less likely to lead to a successful conception.

Researchers found that cannabis use was associated with a higher rate of abnormal morphology – the same trait seen in sperm samples produced during the summertime.

No other lifestyle factors, such as alcohol and smoking, were seen to have an effect.

It is important to point out that only one aspect of sperm quality was analysed. Other aspects, such as sperm motility (how good a “swimmer” each individual sperm is) or sperm count, were not studied.

The study was also reliant on the self-reporting of alcohol, smoking and cannabis use, which is likely to have been underestimated.

Until a more comprehensive look at lifestyle effects on fertility is provided, it is safe to say that choosing a healthy lifestyle is not going to reduce the chances of conceiving and will provide a healthier environment in which to bring up a child.

Where did the story come from?

The study was carried out by researchers from The University of Sheffield, the University of Manchester and the University of Alberta, Canada. It was funded by the UK Health and Safety Executive, the UK Department of Environment, Transport and the Regions, the UK Department of Health and the European Chemical Industry Council.

The study was published in the peer-reviewed medical journal Human Reproduction. The article is open access, meaning it is free to view and download online.

The media has largely ignored the fact that this study only looked at the shape of the sperm and not the sperm count, sperm motility or, indeed, fertility itself.

This may give readers the misleading impression that it has been proven that lifestyle factors such as obesity, smoking and excessive alcohol consumption do not have any negative influence on fertility rates. This is definitely not the case.

Read more about the risk factors for infertility.

What kind of research was this?

This was a cross-sectional study of men attending fertility clinics to see if there was a link between the shape of sperm (sperm morphology) and lifestyle.

The data came from a large study, which has also looked for links between lifestyle and other measures of sperm quality.

Semen analysis looks at numerous factors such as the acidity, quantity and thickness of the semen, as well as sperm quality.

This is assessed by the morphology, concentration (sperm count) and motility (ability to swim). Previous analysis using the same data set looked at concentration and motility, and did not find any conclusive lifestyle risk factors. This analysis only took the shape into account.

What did the research involve?

The researchers recruited 2249 men out of 4257 who had attended fertility clinics in the UK and were eligible for the study. They completed a questionnaire, had an interview on their lifestyle and also undertook a semen analysis. Statistical analysis then looked for links between the lifestyle of those men who had normal-shaped sperm and those who did not.

Men aged 18 and over were recruited to the study from 14 fertility centres in the UK between 1999 and 2002. They were eligible if they had been trying to conceive without success for at least 12 months and had not previously had semen analysis, or did not know the results of any analysis. They also had to understand English.

Exclusion criteria included:

  • having a known medical condition that could cause fertility, such as cystic fibrosis
  • previous treatment than can cause infertility, such as radiotherapy and chemotherapy
  • previous sterilisation of either couple, such as vasectomy or tubal ligation

The men who agreed to take part were given a brief questionnaire to complete at home, which asked about their job, lifestyle and health. They were asked to abstain (from ejaculation) for three to five days and then returned to the clinic. A research nurse then asked further questions about the type of underwear and clothing the person wore, any recreational drug use and fertility history. During the latter part of the study, their body mass index (BMI) was calculated and ethnic group recorded. They were excluded if they did not answer all of the questions.

Semen samples were then analysed. Cases with reduced fertility were defined as having a normal shape in less than 4% of the 200 sperm that were assessed. If there were less than 200 sperm on the slide, they were excluded from the analysis.

They then compared the lifestyle factors of men who had normal shaped sperm in less than 4% of the 200 sperm that were assessed, compared to those who had more than 4% normal-shaped sperm.

What were the basic results?

The study recruited 2249 men, representing just over half (53%) of those who were eligible.

Of those recruited, 173 were excluded because:

  • 81 had no sperm on the slide
  • 47 had fewer than 200 sperm on the slide
  • 43 were contaminated
  • 2 were lost

They further excluded all men who were recruited in the first six months of the study, because during this time there was a very high proportion of men with less than 4% of normal-shaped sperm (54.7%) compared to during the rest of the study (16.1%). The researchers did not know why, so excluded them in case they biased the results.

The analysis included 318 men who had less than 4% normal-shaped sperm out of 200, compared to 1970 who did.

Men who used cannabis in the previous three months were more likely to have poor shape than those who didn’t (Odds Ratio [OR] 1.55, 95% Confidence Interval [CI] 1.04-2.30). The effect was greater in men aged 30 or less (OR 1.94, 95% CI 1.05 to 3.60).

Samples collected in the summer were more likely to be reduced shape compared to those collected in the winter (OR 1.99, 95% CI 1.43 to 2.72)

Samples collected after abstinence of more than six days were less likely to be of abnormal shape (OR 0.64, 95% CI 0.43 to 0.95).

No other risk factors had a significant link to sperm shape.

How did the researchers interpret the results?

The authors conclude that the study “has identified few modifiable factors associated with poor sperm morphology, with the only practical advice to men attempting conception being to limit exposure to cannabis if they are regular users. We would argue that the results of this study, in combination with our papers that investigated the effect of lifestyle and occupation on poor motile sperm concentrations, suggest that men can make relatively few lifestyle changes to improve semen quality, either to enhance natural conception or improve their chances in assisted conception”.


This study has found that the shape of sperm is less likely to be normal when samples are provided in summer and if cannabis has been consumed in the previous three months. It has also found that normal-shaped sperm are more likely to be produced after six days of abstinence. It did not find any other links between lifestyle factors and sperm shape.

The results of this study alone do not show that lifestyle factors other than cannabis are harmless. Male fertility is not just determined by the shape of the sperm; it is also dependent on the concentration, motility and viability of the sperm, as well as the quality of semen. Lifestyle factors could potentially have an effect if they were all combined.

A further limitation of this study, acknowledged by the authors, is that less than half the men attending the fertility clinics met the study's inclusion criteria, and of those who did, only two out of five agreed to participate. The reasons for this are unclear, but could theoretically be because they did not want to disclose their lifestyle.

The men with less than 4% normal sperm shape per 200 were not matched to the “controls” for social and health background data. This is usually conducted so that other confounding factors can be taken into account.

In addition, the study relied on self-reporting in the form of a questionnaire and an interview; it is likely that levels of smoking, alcohol and cannabis use were underestimated.

Whether or not it can be proved that smoking has an effect on fertility, there are many reasons why a potential parent should not smoke. These include the risk of exposure to secondhand smoke for the mother and the risks of a child growing up in a smoking household, such as asthma or even cot death (sudden infant dead syndrome).

Current recommendations on protecting your fertility, such as quitting smoking and moderating your consumption of alcohol, are unchanged.

NHS Attribution