Marijuana for pets?

The plant Cannabis sativa (marijuana) has been used by humans for both medicinal and recreational purposes for thousands of years. The plant and derivatives have been studied extensively in recent years — a PubMed search for “marijuana” yielded over 24 000 references. Most of these studies addressed the psychoactive features of and the adverse effects of marijuana on humans, including detrimental effects on the developing brain, impairment related to driving, and addiction. Although there are more than 480 unique compounds in cannabis the 2 major cannabinoids are delta-9-tetrahydrocannabinol (THC), the primary psychoactive component, and cannabidiol (CBD), the primary nonpsychoactive component (1). Cannabinoids act on various pathways in cells through cannabinoid receptors (CBs) on these cells. There are at least 2 types of receptors: CB1 is present mainly in parts of the central nervous system that affect cognition, appetite, motor activity, emotions, memory but is also found in peripheral tissues including testes, ovaries, prostate, salivary glands; CB2 is found primarily in cells of the immune system. Endogenous cannabinoids use these receptors as part of a signaling system that regulate function of these cells, and have been implicated in several neurological disorders.

In humans, cannabinoids have been shown to have the potential to be valuable therapeutic drugs. They may be of value in treatment of cancer, nausea and vomiting, pain, epilepsy, inappetence, and inflammatory bowel disease (1,2). Cannabinoids are used in several countries in the palliative therapy of patients with cancer because it inhibits some side-effects due to chemotherapy and radiotherapy. In particular they are used to inhibit vomiting and nausea, combat pain, and stimulate the appetite. They are also used for treatment of epilepsy that is refractory to conventional treatments. There are studies in vitro and in animal models that indicate that cannabinoids also possess antitumor activity (2); however, concerns have been expressed that cannabinoids may exert an adverse effect on cancers because of their ability to suppress anti-tumor immunity. A number of synthetic analogs of cannabinoids found in marijuana have been developed in an effort to obtain potent anti-inflammatory activity while avoiding the psychotropic activity of marijuana. These products offer biochemical uniformity, a reliable delivery system, and relief of symptoms. However, some argue that herbal synergy among components of the plant may be important.

In animals, there is much less information on the effects of cannabinoids. Researchers have concentrated on the toxic effects on dogs, in particular. There are less than 60 papers in PubMed on marijuana in dogs and they are almost all on toxic effects — due to accidental ingestion, experimental administration to study toxicity, or ingestion of synthetic cannabinoids. The information on toxicity is likely to gain increasing importance as legal access to marijuana comes into effect in Canada and spreads across the United States. In Colorado, a State with legalized marijuana for medical use, the frequency of marijuana toxicosis in dogs at 2 veterinary hospitals increased 4-fold over a 5-year period (2005 to 2010) (3). The authors found a positive correlation between the increased number of cases of marijuana toxicosis in dogs and the increased number of medical marijuana licenses issued. Dogs are reported to have a higher number of cannabinoid receptors in the brain compared with humans and it has been suggested that they may be more susceptible to the toxic effects than are humans (1). In the Colorado study 2 dogs that consumed baked goods that included butter with concentrated medical grade THC died.

In human medicine it is likely that we will see a massive increase in research on potential therapeutic effects of cannabis and its derivatives (4). There will be more research on growing marijuana with various concentrations of the major cannabinoid components and with more consistent supply of cannabis. More research will also take place in the laboratory and in clinical trials in humans. There are differences in the CB2 receptors and in the metabolism of cannabinoids in dogs compared with humans and veterinary medical researchers should ensure that they become involved in parallel research on the potential value of marijuana for treatment of animal diseases. Marijuana cookies may well find a place in the veterinarian’s arsenal for treatment of epilepsy and the side-effects of cancer.