Sometimes it pays to listen to the experts, but that's something we are increasingly less inclined to do, the prime current example being access to cannabis for pain relief.

The pro-cannabis lobby has become progressively strident, and persistent (suggesting perhaps that those who are arguing for access aren't actually using the stuff, given that persistence is a quality that is rarely associated with smoking dope), but the basis for their argument has always been doubtful, and was last week dismissed by a leading pain specialist.

Professor Milton Cohen told the Australia and New Zealand College of Anaesthetists that there was no reason to be enthusiastic about cannabinoids in the treatment of chronic non-cancer pain. (The difference between cancer and non-cancer pain was not explained).

He saw no evidence that cannabis would revolutionise the field of chronic pain management, and the College's Faculty of Pain Medicine would not support the use of cannabinoids for chronic non-cancer pain until a clear therapeutic role for them had been identified.

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He was concerned that "anecdote and clamour," and "community enthusiasm," had preceded science, creating a culture of false hope based on the "elusive idea of a magic pill" that was driving community misinformation about the value of medicinal cannabis.

He may be too late, of course. Australia's federal government last week legislated for patient access to Australian-grown and manufactured medicinal cannabis, subject to state and territory government regulations. So far access to cannabis-based products in this country has been restricted to individual prescriptions authorised by the Ministry of Health, but public 'anecdote and clamour' has a way of wearing politicians down.

The cause of those who seek access to cannabis is regularly aided by high-profile supporters, who claim to be using the drug to gain the relief that prescription medicines cannot provide.

The fact, as claimed by Professor Cohen, that data that could allow for an informed decision about the effect of cannabis on chronic non-cancer pain is very poor, and that the conclusions have been "over-sold," is unlikely to stem growing calls for access to cannabis.

The compulsory addition of folic acid to bread, in a bid to reduce the incidence of spina bifida, would be even more intrusive, if that's the right word, than fluoridation, in that no DHB would have the ability to exclude their populations.

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He was prepared to countenance trials of medicinal cannabis so its effectiveness and adverse effects might be considered (and understood), a process that would require the establishment of a patient register, similar to what was being done in Israel.

That would be a good idea, although negative results from such a trial probably wouldn't be accepted by those arguing for access to cannabis, if fluoridation can be taken as an example.

Fluoridation is a little different in that it is not only legal but has government backing. Politicians have delegated the job of deciding who gets it and who doesn't to the country's DHBs, which seem to be solidly in favour, citing scientific data that they say proves fluoride is good for children's teeth.

That hasn't satisfied those who interpret the data differently, and who argue that no government has the right to forcibly administer any kind of medication. The practice is well established though.

The compulsory addition of folic acid to bread, in a bid to reduce the incidence of spina bifida, would be even more intrusive, if that's the right word, than fluoridation, in that no DHB would have the ability to exclude their populations. Some 38 per cent of bread made in this country reportedly now includes folic acid, the majority of consumers probably having no idea that they're eating it.

Some women no doubt seek it out in the hope that it will give their unborn children protection, but the principle doesn't seem to bother us.

There has been no great public reaction, in support or opposition, just as we have seemingly grown used to the addition of iodine to table salt as a means of preventing goitre. We are not compelled to eat iodised salt, but most of us do, either in ignorance of the added ingredient or because we just don't care.

The interface between public opinion and science is sharper in other areas, such as a claimed connection between the measles-mumps-rubella vaccination and autism, made all the more confusing by the fact that vindicating scientific opinion can be found by those who say there is a link and those who say there isn't.

For those who are not especially passionate one way or the other, the debate has become extremely confusing.

One school of thought has it that the MMR vaccination is responsible for an unprecedented epidemic of autism, at least in the United States, and that major reductions in the prevalence of some diseases began long before vaccinations were introduced.

Others say vaccination against childhood illnesses, particularly, offers mass protection against potentially calamitous diseases, and that resistance to vaccines in some quarters is reducing their effectiveness.

There can be no doubting the sincerity of both camps, although the anti-vaccine brigade are not above claiming that medical experts who support immunisation are in the pockets of the vaccine manufacturers.

That's a bit of a long bow - try accusing Lance O'Sullivan or Chris Reid of being in the pay of drug companies next time you bump into them and see what reaction you get - although even an honest, medically informed opinion can be wrong, can't it? Your doctor might well be totally sincere, but it's a matter of faith. The patient has no way of knowing whether they are right or wrong.

The place to decide these arguments, however, is the laboratory or via controlled trials in the community. Whether it be medicinal cannabis, fluoridation or immunisation, 'anecdote and clamour' should be discounted. And don't believe everything social media tells you.

The ability these days of every quack and crank to market their views worldwide has not clarified the waters, although some would argue that social media has finally stripped the scientific establishment of its monopoly on swaying public opinion.

Perhaps that partly explains why many people no longer blindly accept what their doctor tells them. That might not be a bad thing - doctors are as fallible as the rest of us - but what we should really worry about is the fact that eventually public opinion, however misguided, has an effect on politicians.

It is politicians, not doctors, who will have the last say when it comes to access to medicinal cannabis (or legal euthanasia, for that matter, but that's another story), even if many of them recognise the thin end of an undesirable wedge when they see one.

There is no rule, after all, that requires public opinion to be well informed, or free of hidden agendas.

That applies particularly to the cannabis debate, those who support recreational access to the drug no doubt recognising medicinal use as a Trojan horse that, sooner or later, will be wheeled through the gates.

There must always be a place for public opinion in a democracy, but the right to express our views must be exercised responsibly, and preferably from a position of knowledge. That's not always easy when expert opinion is divided, as it is in fluoridation.

There appears to be very little division when it comes to medicinal cannabis though, and we should listen to people who, like it or not, really do, or should, know what they are talking about.

'Community enthusiasm' is a poor substitute for scientific evidence, especially when the issue has the ability to change society, and, if the wrong decision is made, to do colossal harm.