June 30, 2017

The smoking of the green

Q: Did you see cannabis is actually better for pain relief than opioids?

A: Not convinced.

Q: But 92%97% of patients said they preferred it! Even if they’d tried opioids! You’re not saying you know better than them, are you?

A: Imagine you had a survey of people who cycle to work, including lots of people who own cars.

Q: Ok.

A: What proportion of those people would say they’d choose to cycle to work instead of driving?

Q: I don’t know, I don’t drive.

A:  Imagine you had a survey of people who cycle to work, including lots of people who have perfectly good bus routes as alternatives.

Q: Ok.

A: What proportion of those people would say they’d choose to cycle to work instead of driving or taking the bus?

Q: A lot.

A: Maybe even 92%97%?

Q: Maybe.

A: The survey was of about 2000 current medical cannabis users. It’s not surprising they say it works.

Q: So it’s not true?

A: No, I think it quite like is true, or at least partly true — not for everyone, but for a lot of people.

Q: Then what’s the problem?

A: The survey provides almost no additional evidence that it’s true. I already thought it was plausible, but anyone who didn’t think that shouldn’t have had their mind changed by the survey.

Q: Ok, what would convince you? Do we need randomised trials?

A: That would be nice.

Q: And a pony?

A: Ok, ok.  More realistically, a cohort study that follows up a group of people starting to use cannabis for pain relief to find out what they end up preferring, rather than starting with people who prefer cannabis.  You could set that up somewhere were medical use was just being legalised, and it wouldn’t cost that much.


Thomas Lumley (@tslumley) is Professor of Biostatistics at the University of Auckland. His research interests include semiparametric models, survey sampling, statistical computing, foundations of statistics, and whatever methodological problems his medical collaborators come up with. He also blogs at Biased and Inefficient See all posts by Thomas Lumley »


  • avatar
    steve curtis

    Wouldnt a randomised trial mean they didnt know if they were getting a medicinal cannabis or not ?
    Very ill people get morphine, doesnt mean we should have California style ‘medicinal heroin’ shops/clinics

    3 months ago

  • avatar
    Ras Gonzo

    People who have too many burgers, booze and coffee need more marijuana’s. But stopping burgers and booze is better.

    Neurological conditions like autism is med-pots preferred application area. Inflammatory and most chronic conditions are usually always lifestyle and diet mediated. Most cannabis use will probably be to reduce symptoms but not heal, much like allopathic medicine in general.

    Most earlier non pre 1900 peoples had a healthy relationship with hemp products. NZ even had a nun, a Sister Mary Aubear who grew and processed cannabis products.She is being beautified by the catholic chrurch for her life of giving to others.

    History and anecdotal evidence is on its side, but not as we know it.

    3 months ago

  • avatar
    Megan Pledger

    Randomised just means the participants get assigned to different arms of a trial based on some randomisation procedure. Blinded means the participants don’t know what treatment they are getting.

    IIRC Sativex is flavoured with peppermint but I think the side effects of marijuana are so well known that it would be difficult to have a blinded trial (even at doses where people don’t get high). But then the same goes for opioids.

    Noone is suggesting “medicinal heroin” shops.

    3 months ago

    • avatar
      Thomas Lumley

      Exactly. The key part of randomisation is that the two groups start off comparable.

      There’s still the possibility for placebo effects, but (a) I don’t actually care in this context, given that cannabis is safer and no more expensive to produce, and (b) placebo effects on pain are probably mediated by the brain’s own opioids and so are less likely to cause bias here.

      3 months ago