Moderate cannabis use can boost men’s sperm count and increase their levels of testosterone, according to new research.
Previous studies had shown that smoking cannabis reduces sperm counts and testosterone levels.
But American researchers found benefits in small amounts of the drug.
Their findings, published in the journal Human Reproduction, showed that sperm and testosterone levels were higher among moderate users than those who never smoke cannabis.
However, the researchers say their results need to be treated cautiously as other factors may account for their findings.
Experts looked at the effect of smoking an average of two joints a week among 662 sub-fertile men in Boston, Massachusetts.
They found that 365 men who smoked cannabis had significantly higher sperm concentration than the 297 men who abstained.
The men taking part in this study completed a questionnaire that asked them about whether they had ever smoked cannabis.
Participants were quizzed if they currently smoked it, the number of joints they smoked a week.
Researchers also found out whether they ever quit and for how many years, the age they started, the last time they smoked cannabis and the total duration of smoking.
The questionnaire asked similar questions about cocaine use, medical history, lifestyle and demographic information.
Men then provided semen and blood samples, which the researchers analysed for information on sperm and reproductive hormones.
Numbers showed that users had 63 million sperm per millilitre of semen compared to 45 million/mL.
This was after adjusting for factors that could affect the result such as age, abstinence time, smoking, alcohol and drug intake.
There were no statistically significant differences in sperm concentration between current and past cannabis smokers.
Among the smokers, users had testosterone levels an average of eight nanograms per decilitre higher than those who used it rarely.
Only five per cent of cannabis smokers had estimated sperm concentrations below 15 million/mL – the World Health Organisation’s threshold for “normal” levels. This is compared to 12 per cent of men who had never smoked the drug.
But the researchers urged caution over their results and called for more studies.
While cocaine use was associated with a higher estimated proportion – 12 per cent having sperm concentration below WHO reference values.
Study author Dr Jorge Chavarro, Associate Professor of Nutrition and Epidemiology at Harvard T.H. Chan School of Public Health, said the drug requires more research.
He said: “These unexpected findings from our study highlight that we know too little about the reproductive health effects of cannabis.
“Also, the health effects in general, to make strong statements about the impact of cannabis on health, with the possible exception of mental health.
“We know a lot less than we think we know.”
The researchers warn that their findings should be interpreted with caution and more research is needed to look at the effects of cannabis use on fertility.
First author Dr Feiby Nassan said the findings were unexpected.
She said: “Our findings were contrary to what we hypothesised at the start of the study. However, they are consistent with two different interpretations.
“First, these findings are consistent with a U-shaped relation whereby low levels of cannabis use could benefit sperm production due to the well-known role played by the endocannabinoid system, but potential beneficial effects reverse at higher levels of use.
“An equally plausible interpretation is that our findings could reflect the fact that men with higher testosterone levels are more likely to engage in risk-seeking behaviours – including drug use.
“So, the relations we see between cannabis smoking, sperm counts and testosterone levels are because men with higher testosterone, within normal levels, have higher sperm counts and are more likely to smoke cannabis.”
The researchers are continuing to study the effects of cannabis smoking in couples and how this could affect the outcomes of treatments for infertility.
Dr Chavarro added: “First, we do not know to what extent these findings may apply to men in the general population.
“Second, since semen quality is a poor predictor of fertility, these findings should not be interpreted as implying better fertility with cannabis use.
“An equally important limitation is the fact that most of the data were collected while cannabis was illegal in Massachusetts, so it is difficult to know to what extent men may have under-reported use of cannabis because of social stigma or potential consequences related to insurance coverage for infertility services.”
By Daniel Hammond