The use of cannabidiol (CBD) to relieve pain in senior horses appears to be safe, but its benefits have not yet been confirmed, according to the authors of a recent study.

The drug might have negligible effects on inflammatory processes and no clear effects on lameness, body condition, or weight in senior horses. Even so, CBD at a relatively low study dose seemed to have no obvious undesired effects either, said Amanda Adams, PhD, associate professor in the Gluck Equine Research Center at the University of Kentucky, in Lexington.

“While it didn’t really do a whole lot, it didn’t hurt,” she said. “At least, we didn’t see any negative side effects in horses.

“Our study was the first to lay the foundation of starting to get a better understanding of the effects of CBD on senior horse health,” she continued. “There are still so many questions that need to be answered before one can say it helps the horse or not really.”

CBD Use in Senior Horses: A “Buzz” Without Evidence?

Already used in humans to treat multiple diseases and their symptoms, including pain, inflammation, and epilepsy, hemp-based CBD has no known psychoactive effects like the related drug Δ-9-tetrahydrocannabinol (THC), commonly found in the annual hemp herb Cannabis sativa L., Adams said. Recently, owners have started treating their dogs, cats, and horses with CBD in hopes of providing them with relief from pain, inflammation, and anxiety, but research is still limited, she added.

“People are using CBD left and right, and I think it used to be more of a buzz, and maybe it still is,” she told The Horse. “People are using it and buying it, without necessarily caring whether there’s science behind it or not.”

Horse owners, in particular, are trying it out on their senior horses, in hopes of giving them more comfort and better welfare, she said.

About 11% of U.S. horses are at least 20 years old, qualifying them as seniors, Adams said. “My lab was the first to characterize that, as horses age—just like humans, cats, and dogs do—there’s this low-grade inflammation called inflamm-aging,” she explained. “So we’ve done quite a bit of work around that. And the thought is that we think we haven’t quite (come to the evidence-based conclusion) to say that this inflammation contributes to age-related diseases in horses such as osteoarthritis. But the thought is that it may. So, if we can modulate the inflammation and bring it down using something like CBD, then it may help the horse.”

CBD as a Treatment for Inflamm-Aging?

Previous study results have shown that CBD can possibly relieve the discomfort of inflamm-aging in human models by reducing pro-inflammatory cytokine production and balancing out oxidative stress, Adams said.

While studies in horses are less advanced, Adams and Shelley Turner, her recently graduated PhD student who focused on CBD research, ran tests in vitro in 2018 that suggested CBD “may have the potential to reduce inflammation in the horse,” Turner said.

“I think that just spurred a lot of people to kind of reach out to me,” she added. “They knew I was looking at different anti-inflammatories and pharmaceuticals that could help lower inflammation in these senior horses and help the health of these horses as they age. So that’s big picture.”

Without proof in living horses, however—because “extrapolating that to in vitro was almost impossible”—Adams and Turner decided to determine, with a new study, CBD’s effects on the immune function of senior horses. In particular, they wanted to know how it would affect inflammatory cytokine production and antibody responses after an influenza vaccine, as well as body weight, body condition score (BCS), lameness, and metabolism.

“We wanted to make sure if we modulated or reduced inflammation with CBD, that it wouldn’t have a negative impact on immune responses to vaccination,” Adams explained.

First Three-Month CBD Study in Horses

Adams and her colleagues divided a group of 27 healthy senior horses—primarily mares averaging 24 years old—into two treatment groups. All the horses lived on pasture at the University of Kentucky’s C. Oran Little Research Farm. Every day, half the horses received an oral dose of 2 mg/kg of CBD derived from a hemp extract, dissolved into approximately 15 milliliters of soy oil, through a syringe. The other horses received the soy oil alone.

The researchers collected blood samples on Days 0, 30, 60, 90, 104, and 111 of their study, just prior to giving the horses either CBD or oil alone, depending on their treatment group. On Day 90, the scientists also took blood samples four hours after the treatment, before all the horses received a commercial equine influenza vaccine.

Few Effects of CBD

Lameness exams carried out 12 days before and 100 days after the start of treatment revealed CBD had no clear effects on forelimb lameness, Adams said. Average body weight and BCS were also unaffected.

The only blood chemistry parameter that changed significantly in the CBD group compared to the control horses over time was albumin (a type of protein which plays a role in circulation), which increased gradually in the CBD horses, Adams said. The control group also had an increase in albumin at 90 days, suggesting external, environmental factors and possibly hormones were at play in the albumin levels at pasture, she added.

On a positive note, the lack of additional biochemistry provides evidence that, at this dose, CBD administered over an extended period does not cause adverse effects on the liver—at least not enough to affect liver enzymes, she said.

Metabolic and inflammatory responses were similar in both groups, she added. “These results may also indicate that CBD does not have a direct effect on lymphocyte cytokine production but may have more of an effect on cytokine production from monocytes, which were not measured in this study,” Turner and Adams stated in their recent publication in Veterinary Immunology and Immunopathology.

On a genetic level, meanwhile, CBD horses had a significant reduction in the expression of IFN-γ on Day 60 and of interleukin-6 (IL-6) on Days 60 and 90. Those reductions didn’t translate into significant effects throughout the study, Adams said.

As far as reactions to the influenza vaccine were concerned, CBD neither increased nor decreased the antibody response. That’s a good thing, because older horses are known to have lower immune responses to vaccinations compared to younger horses, she explained.

“We were glad to see CBD didn’t have any impact on these responses to vaccination,” Adams told The Horse.

CBD Detectable, But Are Effects Resulting From the Dose?

Throughout the study, the researchers were able to detect CBD in the study horses’ blood plasma, Adams said. The lack of clear differences in CBD-treated senior horses might have had something to do with the doses used in the study, she added.

The 2 mg/kg body weight dose might not be sufficient in an herbivore that—unlike cats, dogs, and humans—has a hindgut designed to break down plants in such a way as to potentially affect the absorption and distribution of CBD within the body.

“I think a lot more work needs to be done around the dose and the formulation to make it more bioavailable, with maybe more frequent doses,” she said. “More work definitely needs to be done before we can say whether CBD is good or bad.”

The study, “The effects of cannabidiol on immune function and health parameters in senior horses” appeared in Veterinary Immunology and Immunopathy in March, 2023.

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