There was great hilarity in the Johnston household on Christmas Day when the oldest member of our festive family gathering received an unlikely present: cannabis. Before the Met Police send their drugs squad round to confiscate the spliffs, I hasten to add that this was in the form of something called CBD oil, a compound extracted from industrial hemp which can now be bought legally over the counter in health food shops. Its purpose is to relieve pain, not to get high.

Indeed, cannabis has proven palliative qualities, just as it can have a debilitating impact on the mental wellbeing of some users. Yet so irrational is the debate in the UK around this particular herb that any mention of its beneficial attributes is buried beneath a moral panic about encouraging the “drug culture”.

Elsewhere in the world, the trend is toward legalisation. California this week became the latest US state to allow recreational cannabis use. It has been decriminalised in 21 states and is on sale for medicinal use in 29. Later this year, Canada is expected to legalise the drug for recreational use; it is already available to registered purchasers for medicinal purposes.

Other countries where it is licensed for therapeutic reasons include Australia, Puerto Rico, Poland, the Czech Republic, Canada, Croatia and Macedonia. In Portugal, Uruguay, Spain, Slovenia, the Netherlands, Jamaica, Colombia and Chile it is legal or decriminalised in some form.

In America, though use of cannabis is still technically a federal offence, the drugs administration supports research into the medical use of marijuana and its constituents, acknowledging that it relieves a wide variety of ailments, from epilepsy and multiple sclerosis to cancer.

So, why is it that in the UK we are so unwilling even to have a debate about the subject let alone follow the lead being set by many others? These countries are no less concerned about drug misuse than we are. On pragmatic and health grounds, they have adopted policies that in this country are regarded by some as immoral.

Any attempt to reopen the arguments around cannabis is instantly shut down by politicians who are terrified of being pilloried. Fifteen years ago, as home secretary, David Blunkett made a brave and tentative stab at bringing some balance to the issue by reclassifying cannabis from a Class B to a Class C substance under the Misuse of Drugs Act, thereby reducing the criminal penalties attached to possession.

The criticism was such – not least from the Tories – that within a few years Gordon Brown reversed the policy, not because it was wrong but in order to differentiate his premiership from the Blair years.

Since then, there has been no serious political discussion on the issue, even if, in the real world, the police no longer take much notice of low-level recreational use, which was the intention of the original reclassification. So, we have a law which is not being enforced properly.

Supporters of the current British position usually make two points. One is that modern cannabis is more dangerous than it used to be because varieties have been produced with high levels of THC, which can impair mental capacity and has been associated with schizophrenia and psychotic episodes. The other is that the number of drug users has been in decline, thus demonstrating the success of the criminalisation policy.

It is true that “skunk” and other heavy-duty cannabis products are preponderant now, but if anything that is an argument for regulating sales so that less dangerous varieties are more widely available. It is also the case that drug use has been falling for many reasons, none of which are connected to the Government’s punitive approach.

On the other hand, the harms caused by cannabis have not been mitigated; if anything they are greater due to its additional potency. Successive governments have resisted pressure to adopt a harm-based policy which would place cannabis below alcohol and tobacco in terms of the relative damage it causes. Scientists appointed to act as independent advisers to ministers have been cut off at the knees for making heretical suggestions to politicians who fear appearing to be soft on drugs.

When I sat on the Royal Society of Arts’ drugs commission 10 years ago, the discussion always returned to how the criminalisation of use of soft drugs acted as a barrier to sensible policy considerations. Taking the supply of all drugs out of the hands of criminals, for whom it is a multibillion pound annual business, while regulating content, controlling distribution and taxing sales would be the ultimate reform and is simply not going to happen.

However, as many other countries have concluded, where cannabis is concerned, a more sensible approach is needed and certainly one that allows the drug to be used for medicinal purposes.

Of course, cannabis therapies are not without controversy and there is conflicting evidence on their benefits. However, it is bizarre that the prescription of opioid-based painkillers is considered to be fine, when they are potentially far more dangerous and addictive than cannabis, which is illegal.

This refusal to countenance any change to drug laws is just bone-headed. If the Conservatives are worried about a voter backlash, they should consider that polls show a majority of people favour decriminalising cannabis for medicinal use and there is strong backing for legalisation among young people, whose support they crave.

Why on earth should research into the efficacious properties of cannabis be licensed – at substantial cost – by the Home Office rather than the Health Department? On what possible grounds are people whose pain-racked lives might be eased by cannabinoid products being denied to them? You sometimes wonder whether the objection is to the caricature, louche lifestyle associated with cannabis use rather than anything scientific.

Next month, the Labour MP Paul Flynn – at 82, hardly your archetypal pothead – is promoting a private member’s Bill to allow the production, supply, possession and use of cannabis and cannabis resin for medicinal purposes. In any sane world this would be supported by all MPs and adopted by the Government.

Original post written by Philip Johnston for TheTelegraph