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It is clear from online forum discussions among dog owners that there is interest in the perceived benefits of cannabidiol (CBD). Some dog owners either already give CBD to their dogs, or are interested in trying it, to relieve arthritis pain. Consequently veterinary professionals may be asked for advice about CBD. By doing this module you will:

  • understand what is known about the physiological effects of CBD in dogs;

  • know the legal status of CBD products;

  • be aware of the published evidence on the clinical efficacy of CBD in dogs with osteoarthritis; 

  • be aware of what is known about the safety of CBD in dogs.

Cannabidiol (CBD) is one of many substances (collectively called cannabinoids) that occur naturally in the cannabis plant Cannabis sativa. Cannabis has traditionally been used by humans for its psychoactive effects, which are mainly due to the cannabinoid delta-9-tetrahydrocannabinol (THC). Cannabis used in this way is also known as marijuana. Unlike THC, CBD does not have psychoactive effects.  

Mammals produce substances (which have become known as endocannabinoids) that interact with the same receptors in the body as the plant cannabinoids. This has led to the idea that humans and dogs have an endocannabinoid system.  However, the precise role of endocannabinoids in the body is not yet clear. They appear to be involved in various bodily functions, including the control of appetite, pain perception and memory formation. CBD is used in humans for the treatment of seizures in rare forms of epilepsy, but the mechanism of anticonvulsant action, which appears not to involve cannabinoid receptors, is not known . So far, the exact mode of action that CBD might have in relieving pain is also not understood, but more information is emerging from research. Endocannabinoids have been detected in the joints of dogs with osteoarthritis, although the relevance of this is not yet clear. In rats with experimentally-induced osteoarthritis pain, injection of CBD into the joints was found to reduce pain. 

There are two main pieces of UK legislation affecting cannabis: 

  • The first is the Misuse of Drugs Act 1971. This classifies drugs as Class A, B or C. The class relates to the penalties that apply to offences involving the different drugs, according to how harmful the drug is considered to be when misused.

Class A drugs include cocaine, heroin, LSD and morphine. 

Class B drugs include cannabis, cannabis resin, codeine, ketamine and pholcodine. 

Class C drugs include benzodiazepines (e.g. diazepam) and tramadol.

  • The second piece of legislation is The Misuse of Drugs Regulations. The main purpose of the regulations is to determine the rules about producing, supplying and possessing the drugs that are included under the Regulations. They also state the requirements around prescription writing, record keeping, labelling and safe custody of these drugs. The drugs are listed in one of 5 Schedules. Cannabis and cannabis resin are classed as Schedule 1 drugs, making them illegal to possess or supply. Schedule 1 drugs are considered to have no therapeutic value. This is the reason why research into the medicinal uses of cannabis and its constituents has been inhibited.

Cannabis for medical use (so-called “medicinal cannabis”) is legal in many other countries (Connelly 2018). In these countries the government controls cannabis production, and supply is restricted to regulated outlets (e.g. pharmacies), although some countries allow self-cultivation. Some countries restrict the conditions for which cannabis can be prescribed. The UK government recently announced that it will consider applications from senior doctors for the prescription of medicinal cannabis for specific patients, and that it will review the scheduling of cannabis, but that this will not affect the classification of cannabis as a Class B controlled drug.

CBD does not come under the Misuse of Drugs legislation. However, in the UK, CBD is considered to be a veterinary medicine. The veterinary medicines regulator (the Veterinary Medicines Directorate [VMD]) made this clear on 14 September 2018 and has since written to UK CBD suppliers and manufacturers informing them of the position. This means that without a marketing authorisation it is illegal to sell, supply or advertise CBD products for pets, with or without specific treatment claims. There are currently no authorised CBD products for animals in the UK. 

Products containing CBD that are marketed for medicinal purposes in humans are considered by the human medicines regulator (the Medicines and Healthcare Products Regulatory Agency [MHRA]) to be medicines, and so require a marketing authorisation (MHRA 2016). However, currently there are no products containing CBD alone authorised in the UK for medicinal use in people. There is an authorised medicine containing cannabis extract (which contains CBD and THC) for symptom improvement in people with spasticity due to multiple sclerosis (Sativex oral spray). In the USA there is an authorised medicine containing CBD for use in humans with rare forms of epilepsy (Epidiolex oral solution).

In the UK, some companies market human CBD products as supplements. It is illegal for companies to make specific medical claims for these without a marketing authorisation. These products are unregulated and not standardised. Studies in the USA have found that the amount of active ingredient in commercial unregulated CBD products varies widely, or is often inconsistent with the amounts stated on the label. 

Varieties of Cannabis sativa that contain very little cannabinoids (often referred to as hemp or industrial hemp) have been cultivated for their seed and fibre. These, and the oil derived from seeds, are found in some herbal products, including some marketed for dogs. They may contain little or no CBD.

After administering by mouth, absorption of the raw material form of CBD from a dog’s gastrointestinal tract is poor and unreliable, perhaps because CBD is poorly soluble in water or because it is broken down to a great extent by the liver before it reaches the main circulation. Whether CBD is absorbed in useful amounts might depend on the formulation in which it is given.

Only one randomised controlled trial of CBD in dogs has been published in full (Gamble et al 2018). The trial included 22 client-owned dogs with radiographic evidence of osteoarthritis, signs of pain according to owner assessment, visually detectable lameness, and painful joins on palpation. 

This trial assessed a commercial hemp extract product in an olive oil base containing around 10mg/mL of CBD; the product also contained small amounts of THC and other cannabinoids. The dogs received CBD (at a dose of 2mg/kg) or placebo (olive oil containing anise and peppermint oils to give a similar herbal smell) twice daily for 4 weeks. This was followed by a 2-week washout period after which the dogs were switched to placebo or CBD for a further 4 weeks. Owners and vets were unaware of treatment allocation. 

Among the 16 dogs that completed the trial, the researchers found a small but statistically significant decrease in pain and an increase in activity relative to baseline values at weeks 2 and 4 during CBD therapy but no significant change with placebo. However, there were no changes in veterinary-assessed lameness, pain or weight-bearing. 

The trial has several limitations: a small number of dogs, a relatively short duration for a chronic condition with signs that wax and wane, and it was funded by the company that markets the hemp extract used in the study. It is also important to be aware that these results cannot be applied to other CBD products, which may contain different amounts of CBD. Other studies are ongoing but it is too soon to know whether CBD is effective at relieving pain in dogs. 

In the clinical trial described above, no adverse effects were detected with CBD apart from an increase in liver enzymes, which increased significantly in 9 out of 16 dogs over the 4 weeks of the trial. The effect of CBD on liver enzymes may mean that CBD has the potential to interact with other medicines.

There are no licensed CBD products for animals. A vet may prescribe a human medicine for a pet in accordance with the veterinary prescribing cascade, which allows the clinical needs of an animal to be met with an unlicensed product (VMD 2015). However, there are no licensed human forms of CBD in the UK. CBD supplements marketed for humans in the UK are unregulated. Given that the dose and benefit of CBD in animals is uncertain, it is difficult to see a justification for prescribing a CBD product for an animal. 

There are currently many uncertainties about whether CBD is beneficial for dogs with osteoarthritis, and no suitable veterinary or human products. If CBD is effective in dogs, we do not yet know the optimum dose. Little is known about the adverse effects of CBD in dogs; in one small trial it appeared to be generally well tolerated but liver enzymes were raised in more than half of the dogs while on CBD and there is a potential for CBD to interact with other medications. CBD products sold as supplements for humans are unregulated and so their contents are unstandardised. Research into the therapeutic effects of CBD is ongoing and may be made more possible through future changes in the legislation affecting cannabis.

Podcast

You can listen to a complete podcast of the module by using the play button below or use the download link on the right-hand side of the player to download the podcast to your mobile device.

CBD-test-your-knowledge.jpg

References

Search strategy: We searched PubMed via RCVS Discovery, and VetMedResource using the terms (CBD OR cannabidiol) AND (dog OR dogs OR canine OR canines).

Bonn-Miller MO et al. Labelling accuracy of cannabidiol extracts sold online. JAMA 2017; 318: 1708–9.

Borgelt LM et al. The pharmacologic and clinical effects of medical cannabis. Phamacother 2013; 33: 195–209.

Connelly D. A quick guide to medical cannabis. Pharm J 2018; 301: 18–9.

Gamble L-J et al. Pharmacokinetics, safety, and clinical efficacy of cannabidiol treatment in osteoarthritic dogs. Front Vet Sci 2018; 5: 165.

Grotenhermen F. Pharmacology of cannabinoids. Neuroendocrinol Letters 2004; 25:14–23.

Home Office. Cannabis and cannabis-related medicinal products, September 2018.

Misuse of Drugs Act 1971. [Accessed 25.10.18]

Misuse of Drugs Regulations 2001. [Accessed 25.10.18]

 Medicines and Healthcare Products Regulatory Agency. MHRA statement on products containing cannabidiol (CBD), October 2016. 

Philpot et al. Attentuation of early phase inflammation by cannabidiol prevents pain and nerve damage in osteoarthritis 2017; 158: 2242–51.

In brief. Cannabidiol products should be regulated. Vet Record 2018; 183: 13.

Samara E et al. Pharmacokinetics of cannabidiol in dogs. Drug Metabolism and Disposition May 1988; 16: 469–72.

Stockley’s herbal medicines interactions. Williamson E et al (eds). Pharmaceutical Press, 2013.

US Food and Drug Administration. Warning letters and test results for cannabidiol-related products, February 2017. 

US Food and Drug Admininstration. Epidiolex (cannabidiol oral solution). Highlights of prescribing information, June 2018.

 Valastro C et al. Characterisation of endocannabinoids and related acylethanolamides in the synovial fluid of dogs with osteoarthritis: a pilot study. BMC Vet Res 2017; 13: 309.

Veterinary Medicines Directorate. The cascade: prescribing unlicensed medicines

Veterinary Medicines Directorate. VMD statement on veterinary medicinal products containing cannabidiol, September 2018. 

World Health Organization. Cannabidiol (CBD) critical review report. Expert Committee Review on Drug Dependence. 40th meeting, June 2018.