Evidence Update- New Review of Medical Marijuana Research

I have addressed the question of medical marijuana and the use of other cannabis-derived medicinal products twice before. My conclusion in 2013 was this:

The current research evidence supports a couple of uses in humans, including treatment of nausea and poor appetite and possibly pain. Most other uses are poorly supported by clinical research. And there are unquestionably side effects that make marijuana often less useful than isolated cannabinoids or other unrelated treatments.

There is virtually no useful research evidence in companion animals, so any use of cannabis products is based entirely on theory and extrapolation from the limited research results in humans. Canna-Pet as a specific product, is being marketed with very dramatic and aggressive claims about safety and efficacy that do not appear to be supported by specific research on the product but, again, are based entirely on theory and anecdote, both notoriously unreliable sources of evidence.

There are recognized behavioral and medical risks associated with marijuana use in humans. While the behavioral risks do not apply to use in companion animals, and the medical issues associated with THC do not apply to products with negligible amounts of this compound, the risks of cannabis-derived compounds in dogs and cats are largely unknown. Any use of such products, then, should be undertaken with a clear understanding of the high levels of uncertainty about the results, and claims should not be made for these products that go beyond the available evidence.

When I looked again in 2016, I found no further research in veterinary patients, and the basic bottom line had not changed. We often must extrapolate from research in humans to clinical practice in veterinary medicine because we often don’t have the research we need in our own patients to guide us. This is necessary, but it is also risky. Humans are not dogs or cats, and while there are many similarities in basic physiology and in disease and response to treatment, there are also critical differences. The ibuprofen or sugarless gum that is safe for us will easily kill our pets. And the problems with blood lipids and cardiovascular disease that are a major source of human suffering and death are virtually non-existent in our canine and feline companions. So we must also be aware of the risks of leaping from evidence in humans to evidence in veterinary patients even when we have no choice but to make such a leap.

In the case of marijuana and other cannabis-derived products, there is still effectively no research on the risks and benefits in companion animals. Unfortunately, this has not stopped an explosion of marketing of cannabis products to pet owners, with no testing or regulation to ensure safety and efficacy. This is a dangerous situation. The evidence of risks and benefits in humans can help us to some extent to guess at the effects in companion animals, though unfortunately we cannot know how accurate these guesses will be without doing the careful work of rigorously studying these products in the actual species in which we are thinking about using them.

As for the human evidence, on that front there is some good news. The stigma that has hampered research for so long is waning, which is beginning to open the doors to researchers investigating the real effects of the many compounds found in cannabis. And a new review has just been produced which clearly and comprehensively summarizes the existing evidence, making it a lot easy to see the potential risks and benefits that we should focus on investigating in dogs and cats.

The National Academies of Sciences, Engineering, and Medicine. The Health Effects of Cannabis and Cannabinoids: The Current State of Evidence and Recommendations for Research (2017)

This review does the valuable work of comprehensively searching the research literature and then grading the strength of evidence for specific uses. Here is the grading scale the report uses:

Conclusive Evidence
For this level of evidence, there are many supportive findings from good-quality studies with no credible opposing findings. A firm conclusion can be made, and the limitations to the evidence, including chance, bias, and confounding factors, can be ruled out with reasonable confidence.

Substantial Evidence
For this level of evidence, there are several supportive findings from good-quality studies with very few or no credible opposing findings. A firm conclusion can be made, but minor limitations, including chance, bias, and confounding factors cannot be ruled out with reasonable confidence.

Moderate Evidence
For this level of evidence, there are several supportive findings from good- to fair-quality
studies with very few or no credible opposing findings. A general conclusion can be made, but limitations, including chance, bias, and confounding factors cannot be ruled out with reasonable confidence.

Limited Evidence
For this level of evidence, there are supportive findings from good- to fair-quality studies with most favoring one conclusion. A conclusion can be made, but there is significant uncertainty due to chance, bias, and confounding factors.

No or Insufficient Evidence
For this level of evidence, there are mixed findings, a single poor study or health endpoint has not been studied at all. No conclusion can be made because of substantial uncertainty due to chance, bias, and confounding factors.

This is a reasonable and fairly user-friendly scheme for categorizing the strength of the research evidence for or against specific proposed risks and benefits. The summary of the study lists the specific risks or benefits that were reviewed and classified according to this scheme. The benefits and risks are divided into several categories. There are too many to list here, but I will highlight those for which the evidence is at moderate or better and which might be relevant to veterinary patients (meaning, I will skip uses of cannabis for diseases not seen in dogs and cats or risks, such as auto accidents or risks from smoking marijuana, which obviously don’t apply to these species.)

Therapeutic Benefits:
Treatment of chronic pain in adults (cannabis)
Treatment of chemotherapy-induced nausea and vomiting (oral cannabinoids)

Risks:
Lower birth weight with maternal use
Impairment of cognition, including learning, memory and attention with acute use
Variable increase in risk of development or in symptomatic worsening of various psychiatric disorders (relevance to behavior problems in veterinary patients?)

That’s pretty much it. There are plenty of other risks and benefits with at least moderate evidence that appear only to be relevant to humans. And there are many risks and benefits which could be relevant to veterinary patients but for which the evidence in humans is weak or insufficient to draw any conclusions. And, as I keep pointing out, there is virtually no evidence for any of the many different products out there that directly evaluates risks and benefits of those products in dogs and cats.

So right now, we are at a place where the hype and the marketing far exceed the real evidence that cannabis-based products are safe and useful for our pets. Using them could be worthwhile, but currently it is essentially rolling the dice, an uncontrolled individual experiment that could also make the patient’s life worse. Hopefully, further research will elucidate the real harms and benefits for our pets so we can make sound decisions about the appropriate role of cannabis-based products in veterinary care.

 

 

 

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3 Responses to Evidence Update- New Review of Medical Marijuana Research

  1. anja hunter says:

    I just discovered your website. I love it! Thank You

    From your fellow skeptic Dr Hunter

  2. DL says:

    Thanks for your blog! I’m a pet owner, and I’ve noticed that a number of local vets are into various practices that I’m sceptical of. (I’m from Canada; that’s how we spell sceptic.) Despite my doubts, I just tried hemp oil for my dog’s chronic rhinitis; no effect. Back to proven Western meds.
    It was great to see you quoting the Cochrane folks! Carry on!

  3. DL says:

    PS – I got the idea for trying the hemp oil from Dr. Silver’s website, not from my vet.

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