Evidence Update: Discouraging News for CBD & Arthritis in Dogs | The SkeptVet

I have been tracking the growing body of evidence concerning veterinary uses of CBD since 2018, and it is an exciting and fast-changing field. While there has only been a little clinical trial research in dogs and cats, there have been a couple of encouraging papers looking at CBD for treatment of canine arthritis.

A 2018 study looked at 16 dogs taking either 2 mg/kg of CBD or a placebo twice daily and evaluated subjectively by blinded owners and veterinarians. The owner measures showed improvements in pain and function, while the veterinarian measures showed improvement in pain but not function.

A second study in 2020 evaluated 20 dogs in four groups: high-dose CBD (50mg/day), low-dose CBD (20mg/day), liposomally encapsulated CBD, and placebo. Generally, owner and veterinarian subjective measures improved for the high-dose and liposomally encapsulated CBD groups and not for low-dose CBD or placebo, though there was some variation.

These studies are small and have some significant limitations, but as preliminary evidence, they are encouraging, and I have been hopeful that eventually the data will be robust supporting the use of CBD for arthritis pain in dogs. However, a new study has dampened that hope somewhat.

Mejia, S. et al. (2021) ‘Evaluation of the Effect of Cannabidiol on Naturally Occurring Osteoarthritis-Associated Pain: A Pilot Study in Dogs.’, Journal of the American Animal Hospital Association. J Am Anim Hosp Assoc. doi: 10.5326/JAAHA-MS-7119.

In this study, 23 dogs were randomly assigned to begin treatment with 2.5 mg/kg twice daily of hemp oil with CBD or hemp oil without CBD, and then were crossed over to the other treatment after 6 weeks. Any effects were assessed by activity counts (essentially Fitbits for dogs), weight bearing using a pressure mat, and evaluation of gait by a blinded veterinarian.

Blood testing showed that dogs on the placebo had no measurable CBD or THC in their blood while dogs getting the active treatment had both. Unfortunately, no difference between the treatment and placebo was identified by any of the measures evaluated. As is common in studies of CBD, a few dogs had elevations of liver enzymes without apparent clinical symptoms on the drug, and a couple experienced vomiting when taking it, one of whom had serious enough symptoms to be removed from the study. A small placebo effect was detected by one measure of effect, but not by the others.

As always, individual studies are almost never the definitive word on any medical question. Just as the previous studies were encouraging but by no means absolute proof that CBD is helpful for arthritis in dogs, this study is discouraging, but it does not close the book on CBD as an arthritis treatment. There are several possible explanations for the lack of an effect in this study. The most obvious is that CBD doesn’t help dogs with arthritis, and that is certainly a possibility. However, it is also possible that no effect was seen because of the specific product or dose used, the lack of a washout period between CBD and placebo, the effects of other chemicals in the hemp oil or other treatments the dogs were receiving, or issues with the measurement tools used.

The presence of adverse effects (vomiting and liver enzyme elevations) as well as the difference seen in blood CBD levels and the presence of some caregiver placebo effects are all good signs in that they indicate the study was using a treatment with significant biological effects and measurement tools that could distinguish, at least in part, between placebo effects and true treatment effects. Studies that show no adverse effects and no placebo effects raise a red flag that there may be something wrong with their methods or the treatment being tested.

This study also raises the issue of the Decline Effect. This is a phenomenon in science in which early research into a novel idea tends to show strongly positive findings. However, as more studies are done by a greater variety of scientists, the size of the effect tends to decline, sometimes to zero. This likely represents both the influence of bias in early studies caused by enthusiasm of scientists for their own new hypotheses and a gradual improvement in methods and reduction in overall positive bias as the scientific community studies new ideas over a period of time.

It may be that the different findings of individual studies looking at CBD as a treatment for arthritis in dogs result from specific differences in study methods or the CBD products used. Or it may be that we are seeing the beginnings of the Decline Effect in action, and that there will turn out to be little benefit to this treatment, or less than we initially thought. The most important thing is that we continue to study the issue, refining our hypotheses and methods as we go along. This path provides the best chance of finding the real value, if any, of CBD for our arthritis canine companions.

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