Men who have smoked cannabis at some point in their life have “significantly higher” sperm counts than those who have never used the drug, according to new research.
Scientists found there was no significant difference in sperm concentrations between current and former smokers.
Dr Jorge Chavarro said the findings were unexpected – and highlight how little is known “about the reproductive health effects of marijuana, and in fact the health effects of marijuana in general”.
However, the associate professor of nutrition and epidemiology at the Harvard Chan School also warned that the results “need to be interpreted with caution”.
Feiby Nassan, the study’s lead author, said the findings were contrary to what the team hypothesised.
She also explained they are “consistent with two different interpretations” – the first being that low levels of marijuana use could benefit sperm production.
“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 smoking marijuana,” Dr Nassan added.
The team’s study has been published in the journal Human Reproduction.
Cannabis has now been legalised in a number of US states, amid a growing perception that the drug poses little risk to health.
The researchers expected that smoking cannabis would be associated with worse semen quality, as historical studies had suggested the drug has negative effects on reproductive health.
Many of the older studies had focused on animal models or had examined men with histories of drug abuse.
In the new research 1,143 semen samples were collected from 662 men between 2000 and 2017.
On average, the men were 36 years old, white and college educated – and of the participants, 55% reported having smoked marijuana at some point in their lives.
The researchers have cautioned that there are limitations to the findings, as some of the participants may have under-reported their cannabis use.