Cannabis-derived products may have therapeutic potential, but scientific evidence supporting their use in animals remains limited, according to researchers.
The authors of a just-published review caution that few well-controlled studies have explored the use of cannabis-related products in animals.
Nancy De Briyne and her colleagues, writing in the journal Animals, said veterinarians are seeing more cases of cannabis toxicosis in pets as cannabis-based products have become more available for human use, both medical and recreational.
Animal owners are showing increasing interest in giving these products to their pets, they said. “These clients understandably are asking, ‘Are these products legal, safe, and effective for treating medical conditions in animals?’.”
In their paper, the international team set out to consider the different types of cannabis and cannabis-derived products, and the questions currently arising around their use in animals.
Cannabis and its related products are derived from the plant Cannabis sativa. Most research to date has focused on the biologically active cannabinoids (CBD) it contains. More than 100 different cannabinoids have been identified.
The main psychoactive substance in the plant is tetrahydrocannabinol, or THC. The tightest controls center on products containing THC.
The authors noted that the use of cannabis-related products in horses and humans is not new.
Early Greek writers reported the use of cannabis for dressing sores and wounds in humans and horses. The dried leaves were used against nosebleeds and the seeds against tapeworms. The green seeds were steeped in a liquid such as water or a variety of wine, pressed out, warmed and then instilled into the ear for pains and inflammation associated with blockages.
The Berlin Hippiatrica, a collection of horse remedies, reveals that chopped leaves were used to dress a wound. Another collection, the Cambridge Hippiatrica, offers a recipe for the treatment of tapeworms for horses.
Until relatively recently, cannabis was found in a large number of veterinary medications designed to treat colic, spasmodic colon, and other horse ailments. The bottles of some of these veterinary drugs survive to provide evidence of the therapeutic uses of this plant.
In the 1800s, American veterinarians routinely prescribed equine colic medication which contained high doses of marijuana. The Parke Davis pharmaceutical company was one of the leading suppliers of top quality “liquid cannabis” in the US.
The company, having collected hemp seeds from India and Nepal at the start of the 20th century, began growing high-grade marijuana in Michigan and the Blue Ridge mountains. The US Department of War had no hesitation in recommending its use for colic. Its 1915 Manual for Farriers, Horseshoers, Saddlers and Waggoners recommended giving colicky horses “one teaspoon of liquid Cannabis Americana mixed with one tablespoon of olive oil”.
The review team noted the growing relaxation of laws around cannabis production and use around the globe.
Although the legal cannabis market in Europe is targeted strictly towards medical consumers, the consumption of hemp-derived non-psychoactive CBD-infused products for non-medicinal purposes is legally permitted across much of the world.
The current market size for CBD in Europe is about €450 million, representing 31% of the global CBD oil market share.
“There are many companies in the European Union marketplace today selling ‘nutritional supplement’ cannabis-derived products for dogs, cats, and horses, some of which make what clearly appear to be therapeutic feed claims.
“These products are being promoted as aids for itching, anxiety, nausea, poor appetite, seizures, cancer, digestive problems, inflammation, immune disease, and reduced mobility due to joint pain in animals.
“It is against the law to make therapeutic feed claims about nutritional products.
“Veterinarians,” they said, “cannot offer scientific advice on the effectiveness of a nutritional product to treat a disease, as it is not a medicine and such claims are (a) illegal; (b) unproven; and (c) potentially unsafe.”
No health claims relating to hemp or CBD products are authorized under EU regulations.
There are no authorized cannabis or cannabis-derived veterinary medicinal products on the market in the EU, US, or Canada, they said.
On the EU market, one product is authorised in Germany as a homeopathic veterinary medicine, while one CBD product is registered in several EU nations as a feed supplement.
“The off-label use of human medicinal products might be allowed to be used in animals in certain EU countries or in the US, only when using EU or FDA-approved products, respectively,” the review team said. “It is the responsibility of the veterinarian to understand their legal obligations.”
They said the available scientific evidence on their use in animals is currently limited and focused on companion animals and horses.
“Some findings from a few well-controlled clinical studies have been published, other information is gleaned from anecdotal support, historical records, and case reports or has been extrapolated from studies related to use in humans, including the study of animal models for that purpose.”
Areas of interest include its use for pain, immune-mediated and inflammatory allergic disorders, cardio-vascular and respiratory conditions, and epilepsy.
According to the scientific literature review, CBD products are mainly used in the treatment of pain, especially osteoarthritis pain.
It is difficult, they said, to estimate the effectiveness of using cannabinoids in animals.
“There is also interest in using CBD and other cannabis-derived products for horses, e.g., against arthritis and negative stereotypical behaviours in horses,” the review team noted.
However, the International Equestrian Federation (FEI) has declared all cannabinoids as banned substances on its Equine Prohibited Substances List. Products that cannot be given to competition horses include natural and synthetic cannabinoids and other cannabimimetics.
The authors cautioned that CBD oil products may be marketed that do not conform to regulations, both animal and human.
In the US, the Food and Drug Administration has also had to cite multiple companies illegally selling CBD products because the companies claimed they could prevent, diagnose, treat, or cure disease, with some of these companies marketing products targeted toward animals.
“The marketing of illegitimate products occurs, particularly when the regulatory framework is not well known and during periods of the rapid expansion of any particular sector,” the authors noted.
“In all regions, the inaccurate labelling of the identity and strength of active ingredients is of particular concern, making administration according to a dosage regimen very difficult to impossible.”
They noted that few studies have been done on the safety and tolerability of these products in animals.
“Several studies have shown quality and safety issues related to the use of products, often illegal, labelled for animal use, some of which have been associated with toxicoses.”
The review team noted that veterinarians were seeing more cases of toxicosis in animals arising from the use of cannabis-derived products.
Veterinary cases of cannabis toxicosis in dogs stem most commonly from exposure to edibles. Cats may also directly consume the plant material.
“The likelihood of pets becoming exposed is increasing as cannabis and cannabis products become more widely available and recreational drug use more commonplace.
In 2019, the Animal Poison Control Center run by the US-based ASPCA animal charity noted a large jump in calls about marijuana ingestion by animals.
There is currently little research performed on thresholds for toxicosis in animals.
“Although the available data suggest that CBD may be well tolerated by animals and produces few side effects, the lack of industry-wide quality control can result in an animal’s exposure to hemp or CBD products contaminated with THC or toxins, such as heavy metals or pesticides, that may cause harm.”
In the US, animal poison control organizations indicate that up to 50% of pets exposed to products labelled as CBD or hemp may develop clinical signs severe enough to require veterinary intervention, indicating that such products may not be pure CBD.
“Several deaths were reported to have been related to cannabis toxicoses, and these appear to be the result of associated complications, such as aspiration.”
Because no antidote has been described to date, the treatment of cannabis toxicosis consists of supportive care. Because of the wide margin of safety of most known cannabinoids, toxicosis is rarely fatal.
The review team called for more research to improve the understanding of the safety and effectiveness of cannabis-derived products in veterinary medicine.
“Current research is limited, mostly done on small samples and at times with conflicting outcomes.” Research thus far suggests that cannabinoid products may potentially be beneficial in certain cases to reduce pain, particularly osteoarthritis pain, and as an adjunctive treatment of canine epilepsy.”
Currently, no cannabis-derived veterinary medicinal products are authorised in the EU or North America.
The authors said well-controlled clinical trials were required in support of EU and North American approval for suitable cannabis-derived products. This would ensure that high-quality products of known safety and efficacy can be made available for veterinarians and their patients.
“Clinical trial studies should be encouraged to investigate the potential therapeutic value and safety of hemp-derived products for companion animals.”
There should be a ban on producing pet food with cannabis-derived products without known safety and efficacy and without the knowledge of the intended purpose of the included cannabis-derived products, as specified by the manufacturers.
There should also be a prohibition on producing feed supplements or beddings for food-producing animals with CBD/cannabis without known safety and efficacy data.
“We encourage veterinarians to act cautiously, as there may be risks associated with having such products in their possession if the product(s) were subsequently shown to contain illegal levels of THC.”
They said greater international cooperation is needed to help define standards, promote safety, education, research, and policy around the use of cannabis-derived products.
The review team comprised De Briyne and Sarah Moody, with the Federation of Veterinarians of Europe; Danny Holmes, with Holmes St Anthony’s Veterinary Hospital in Ireland; Ian Sandler and Enid Stiles, with the Canadian Veterinary Medical Association; Dharati Szymanski, with the American Veterinary Medical Association; Stephan Neumann, with the Institute of Veterinary Medicine at the University of Goettingen in Germany; and Arturo Anadón, with the Faculty of Veterinary Medicine at the Complutense University of Madrid in Spain.
De Briyne, N.; Holmes, D.; Sandler, I.; Stiles, E.; Szymanski, D.; Moody, S.; Neumann, S.; Anadón, A. Cannabis, Cannabidiol Oils and Tetrahydrocannabinol—What Do Veterinarians Need to Know? Animals 2021, 11, 892. https://doi.org/10.3390/ani11030892
The review, published under a Creative Commons License, can be read here.