The most senior figure in the NHS has called on politicians to resist growing pressure to legalise cannabis, warning it would have dangerous consequences for young people.
Simon Stevens, chief executive of NHS England, intervened on Tuesday after former Tory leader William Hague said the war on the drug had been "comprehensively and irreversibly lost" and it was time to change course.
Prime minister Theresa May is coming under pressure to review the law around cannabis, after a 12-year-old boy with severe epilepsy was initially denied access to cannabis oil used to alleviate his seizures.
The Home Office granted an emergency licence for the product to be returned to Billy Caldwell, after it was seized by officials at Heathrow Airport last week.
But Stevens warned that it was wrong to confuse the two issues of licensing cannabis products for medicinal use and legalising it for recreational purposes.
Speaking at the launch of a major review into the NHS by former health minister Ara Darzi for the IPPR think tank, attended by BuzzFeed News, Stevens said the government must make sure it did not "inadvertently introduce new risks for our young people".
He said there were "clearly very strong reasons" why experts should look at which cannabis-derived compounds should be prescribed by doctors for conditions such as multiple sclerorisis and chronic pain.
"But I think it's very important as a country that we don't confuse this debate around specifically prescribable products for certain medical conditions with a much more generalised debate around the decriminalisation or legalisation of marijuana, without at the same time reminding ourselves that there are some genuine health risks there," he said.
He said that in those countries where marijuana had been decriminalised, young people often came to think of it as safe – "whereas, let's be clear, actually it isn't".
It increases the risk of long-term psychological problems such as depression or psychosis and also poses "significant risks to your lungs", he warned.
"So let's have that discussion around the prescribable, carefully evidence-based use of these compounds, but let's not forget at the same time that doesn't have to be the generalised debate about whether or not marijuana is safe for teenagers – because on balance, it isn't."
Number 10 also rejected Hague's proposal, saying "cannabis can cause serious harm when misused", while the Home Office confirmed it had no intention of reviewing the classification of cannabis, which would remain Class B.
But Tory MP Crispin Blunt and Liberal Democrat leader Vince Cable were among those who welcomed the intervention, saying the government's policy on cannabis had failed.
Hague wrote in the Daily Telegraph that the case of Billy Caldwell "provides one of those illuminating moments when a longstanding policy is revealed to be inappropriate, ineffective and utterly out of date".