CANNABIS could reduce the size of a man's testicles and impact sperm quality, a study claims.
THC — the psychoactive chemical in the drug — hits male fertility but changes can be reversed when users stop taking it, scientists found.
They gave monkeys increasing doses of the drug over seven months and measured changes to their genitals and swimmers.
Their testes halved in size and sperm quality worsened, thanks to lower testosterone levels.
But four months after exposure was stopped, these effects were partially reversed, the team said.
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Dr Jamie Lo, of the Oregon National Primate Research Center, said understanding the drug’s effect on fertility is vital as more US states seek to legalise it.
He said: “We can now more confidently assure patients that by abstaining from THC for at least four months, the impacts of THC on male fertility can be partly reversed.
“This allows for more concrete, informed recommendations for patients who are in the process of family planning or actively trying to conceive.”
Around 14 million Brits have used cannabis, while nearly 50million Americans did it in 2019, the latest date data is available for.
It is an illegal class B substance in the UK but has been made legal in 21 US states.
A previous study by the researchers showed THC exposure reduced fertility in male rhesus macaques.
The latest study, published in Fertility and Sterility, aimed to see how the animals recovered after the drugs were stopped to see if the issues are reversible.
They found extra 1mg of THC given to a monkey per 7kg of its body weight each day, their testicles dropped in size by 12.6 cubic cm.
But after the monkeys were not given any THC for four months, their testicles inceased back to 73 per cent of their original size.
Dr Jason Hedges, of the ONPRC, said: “We understand that for teens and young adults, family planning might not be top of mind.
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“However, THC even in moderate doses could impact their fertility outcomes, so this is a serious concern for us as healthcare providers.
“The more we can understand and define this issue, the better information we can provide to patients to be able to optimise their reproductive health.”
