Publication bias with respect to negative data in clinical studies is a well-known phenomenon (i.e. negative data is not published). Additionally, current research in this field is limited, with outcomes that are published detailing conflicting findings and being based on small sample sizes (Table 2). With that stated, our review of the literature detailed below generally shows a positive outlook for the future of CBD use in veterinary medicine. Further insights into how CBD can be used appropriately are crucial for optimal use of these compounds (Parker 2022).

The US Food and Drug Administration (FDA) state “FDA is aware of some cannabis products being marketed as animal health products. We want to stress that FDA has not approved cannabis for any use in animals, and the agency cannot ensure the safety or effectiveness of these products. For these reasons, the FDA cautions pet-owners against the use of such products and recommends that you talk with your veterinarian about appropriate treatment options for your pet”. The FDA also states that it has not directly received any reports of adverse events associated with animals given cannabis products; however, a study published by the University of Guelph found that the number of reports to the US animal poison control centre (APCC) of dogs with cannabis poisoning is rising (Howard-Azzeh et al. 2021). Nonetheless, as CBD products gain popularity in the human supplement market, naturally the potential for application and market growth has transferred to companion animals. In 2017, US$ 8 million of CBD pet products were sold in the US, rising to $560 million by 2021 (Shahbandeh 2024b). As with the human market, North America holds the top spot for CBD product sales with Europe close behind, expecting sales to double to US$142 million by 2024 (Shahbandeh 2024b). The CBD pet care market is split between dogs (65%), horses (22%), cats (8%), and others (5%) (rabbits, small rodents and birds) (Shahbandeh 2024c). Despite the size of the pet CBD market, no licensed products are currently available for veterinary use, as there are insufficient data surrounding safety and dosage rates (VMD 2022).

In the US, it is illegal for vets to prescribe, dispense or recommend cannabis or its products for animals (AVMA 2019). However, in the UK and EU, vets are permitted to prescribe CBD products to pets through the cascade (a legislative provision in the Veterinary Medicines Regulations that allows a veterinary surgeon to prescribe unauthorised medicines that would not otherwise be permitted), although it is an offense for CBD products to be given to pets without a prescription (VMD 2022). As in humans, dosage behaviour means that owners administering CBD products to their pets are likely to guess dosage based on insufficient evidence and risk adverse reactions. There is also a concern that CBD may display “bell shaped dose response curves” in that too much of the drug has lesser or no effect compared to the optimal dose. The idea that safety in humans provides data for safety in companion animals is not supportive of licencing due to higher toxicity risk to THC in pets. Furthermore, unlicensed products go through much less rigorous testing, if any, and they may contain contaminants toxic to pets (e.g., high selenium levels); currently there are no proven benefits associated with these products (Summerhill Vets 2024).

Pet owners across the US, EU and Canada have been surveyed regarding their use of CBD products for their pets (Kogan et al. 2019; Tomsič et al. 2022; Corsato Alverenga et al. 2023). In all cases, the majority of owners obtained their information from online sources, with a much lower proportion consulting veterinarians. As a result, there appeared to be a lack of knowledge regarding safe dosage rates and what type of CBD they were administering to their pet. Across the studies, most owners reported using the products mainly as supportive therapies, followed by cases of resorting to CBD when conventional treatments failed. Approximately 1 in 20 British pet owners gave CBD products to their pets in 2022 (Hodges et al. 2022), and approximately 28% of US owners have previously administered CBD to their pet (Corsato Alverenga et al. 2023). It can be argued that this situation is not favourable for optimal treatment strategies for companion animals and value for money for the pet guardian. Further research on patient benefit with respect to conditions in ageing is needed to overcome such concerns.

However, within the present market this is not easily achieved. There are thousands of cannabinoid-containing products available in the pet care market in the forms of oils, edibles, and topicals. Given that CBD is not licenced for medicinal use, the products are sold adhering to packaging and labelling laws: for example, all CBD products marketed for animals need to be marketed as ‘supplements’ to differentiate them from medicines; it must state on the packaging that the product contains CBD, as what compounds, and how much (mg/kg); certain claims such as ‘preventative’ cannot be made unless the product is marked as medicinal; and lastly, the label must provide recommended dosage levels and a toxicity warning (GOV UK 1996).

There is concern regarding the quality of available CBD products on the market, especially for pets. A 2022 review of the cannabinoid market indicated that consumers’ biggest concerns are about the product purity (42%), price (38%), whether the product is Food Standards Agency (FSA) approved (38%), and poor labelling (31%) (Hodges et al. 2022). Additionally, information deemed to mean that the product is a trustworthy item to consumers in descending order are price, inclusion of natural ingredients, clear labelling of ingredients, and clarity on product origins. Purity, pricing and labelling of over-the-counter CBD products is currently un-regulated, therefore, many products do not meet consumer expectations (Table 3), though the market continues to flourish. As CBD is a (complementary) medicinal product in the UK and EU it requires a Veterinary Medicines Directorate (VMD) or European Medicines Agency (EMA) marketing authorisation to be prescribed (Wessmann et al. 2021). Without this, selling, supplying, or advertising CBD-containing products to pets is an offense.In the UK, CBD products are prohibited for use for pets altogether, so much so that online retailers have been forced to cease sales within the UK. Nevertheless, CBD products are still purchased by UK customers. Evidence-based veterinary medicine data does not seem to influence consumer appetite for products in some quarters (Tomsič et al. 2022; Holst et al. 2024).

It is recommended to ask for veterinary advice on the use of CBD products, however, in the US it is illegal in some states for vets to advise on prohibited substances such as CBD. Hemp is the only source legal to supply CBD in the US. Previously, hemp and CBD were federally illegal in the US as pet food ingredients (treats included) though changes to the Farm Bill (2018) moved hemp from schedule 1 to schedule 2 under the Controlled Substances Act to facilitate research in dogs (Corsato Alverenga et al. 2023). Additionally, as all states have their own individual laws about CBD products; this causes further legal issues and barriers for selling between states. Nonetheless the market remains buoyant for use of CBD products in companion animals, with vendors advertising, often appropriately, in making no claims for CBD beneficial effects.

It can be argued that there is a market opportunity for products that are based on hard scientific research with demonstrated benefit in a CBD product, that outstrips the conjecture often found associated with other products. Validity based on veterinary medicine research would certainly meet the consumer concerns and aspirations detailed above.

Tolerability of CBD in companion animals

Recent studies suggest that a daily dose of 4 mg/kg/day is well tolerated in healthy dogs and cats (Bradley et al. 2022; Coltherd et al. 2024). Evidence in the literature has shown that CBD administration to dogs is generally safe and well tolerated for short term use. Adverse effects such as loose stools and vomiting have been documented to affect 0.45–3.3% of dogs receiving CBD (Deabold et al. 2019; McGrath et al. 2018). Studies looking at serum chemistry in dogs largely report no significant alterations, but some have highlighted that liver enzymes, such as alkaline phosphatase (ALP), can become elevated (Gamble et al. 2018; McGrath et al. 2019; Deabold et al. 2019). Due to the complex nature of ALP level regulation, and the fact that age, stress, and certain medications (e.g., corticosteroids) can increase ALP levels, this warrants further investigation. Furthermore, without a concurrent significant rise in alanine transaminase (ALT) to support an indication of hepatocellular damage, the significance of these observations is uncertain. Consequently, controlled long term studies should be inclusive of a protocol for monitoring blood borne liver enzymes (Gamble et al. 2018).

One of the main safety concerns in companion animals appears to be the inclusion of THC. In randomized controlled trials, THC containing compounds produced more severe side effects in dogs than those in the control group, when tested for safety in escalating doses. In contrast to this CBD side effects were found to be mild even at high doses (640 mg) (Vaughn et al. 2020). THC causes toxicosis in dogs and cats and may present clinically as urinary incontinence, disorientation, hyperesthesia, bradycardia, diarrhoea, respiratory depression, twitching, and vomiting (Amissah et al. 2022). As THC is metabolised slowly by the liver of pets, clinical signs may only appear several hours post-ingestion (Chicoine et al. 2020). Concerningly, one US study found the most common source of toxicosis-causing compounds by vets are products from government-regulated producers (Amissah et al. 2022). In this review we are concerned with CBD which generally appears to only have mild side effects in dogs. Administration of CBD oil (62 mg/kg), THC oil (49 mg/kg), and a CBD: THC blend (12:8 mg/kg) were tested for safety in dogs, with all test groups showing mild adverse effects (lethargy, hypothermia) and some showing severe effects (neurological symptoms) (Vaughn et al. 2020).

Safety and tolerance data are largely un-reported in cats, with one study published in 2021 observing mild side effects (hypothermia, lethargy) when testing maximum doses of CBD (30.5 mg/kg), THC (41.5 mg/kg), and CBD: THC blend (13:8.4 mg/kg) (Kulpa et al. 2021). Another study tested doses of CBD oil ranging from 2.5-80 mg/kg where side effects (head shaking, excess salivation) were also found at levels ≤ 40 mg/kg (Rozental et al. 2023).

Overall, to establish a market for a scientifically backed CBD-based product range, there is a definite need for further research into the use of CBD in both humans and animals to establish safety, tolerance levels, toxicity, optimum routes of administration, and further information regarding the metabolism and interactions within the body (Fig. 1).