Don’t Fear the Reefer - Marijuana Toxicity

Nathan Bodnar, DVM, explains the toxic effects of THC (found in marijuana) on dogs and cats. Toxic doses, clinical signs, treatment plans, and patient monitoring are all discussed.

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Dietary indiscretion and resulting toxicities are some of the most common emergencies in veterinary medicine. Each class, and potentially each individual toxin, may present as a distinct set of clinical signs. Marijuana is one of these toxins that can be diagnosed based on the set of clinical signs.

Marijuana, also known as grass, hemp, Mary Jane, MJ, pot, and weed, is one of the most common illicit drugs used and thus, is potentially a very common toxicity. The frequency of marijuana toxicity is most likely to increase with potential decriminalization and increased medicinal use. 

Marijuana is made from the dry leaves and flowers of the hemp plant (Cannabis sativa). The active ingredient is delta 9-tetrahydrocannabinol (THC). THC is found in all parts of the plant, but its concentration is highest in the flowers and leaves. The average marijuana cigarette contains 150mg of THC. Hashish, which is derived from resins of the flowering tops of the plant, contains a higher concentration than the dry plant itself. Hash oil is the most highly concentrated form of THC.

THC interacts will all major neurotransmitters in the brain including norepinephrine, dopamine, serotonin, and acetylcholine and also binds to specific receptors in the cerebellum and frontal cortex.

Marijuana is rapidly metabolized by the mixed function oxidase system of the liver. Between 65%-90% is excreted through the feces, and thus enterohepatic cycling is significant, while 10-25% is excreted through the kidneys.

The LD50 has not been established in dogs. In rats the LD50 is 666-1000mg/kg; thus, marijuana appears to have a wide safety margin.

Animals can be exposed to marijuana in several forms including second hand inhalation of the smoke and oral consumption. Dogs can also consume the dried plant or cigarette directly. Dogs are especially likely to ingest the marijuana when it is used in food products including brownies, cookies, cakes, and butter.

Clinical signs of marijuana intoxication mainly include: neurological signs of depression/sedation, ataxia, disorientation, hyperesthesia along with mydriasis, bradycardia, and urinary incontinence. GI signs including vomiting are also often appreciated in 30% of cases. Large exposure can lead to stupor, hypothermia, and hypotension.

The on set of clinical signs can range from five minutes to 96 hours, but the majority will occur between 1-3 hours after ingestion. Clinical signs can last anywhere from 30 minutes to 96 hours. THC is stored in the body’s fat deposits and thus, can lead to prolonged clinical signs.

Marijuana intoxication can often be easily recognized by the classic clinical signs of sedation, ataxia, hyperesthesia, and urinary incontinence. Definitive diagnosis is usually achieved through proper history with known ingestion. There are several human urine drug screening kits that can test for THC but can give false negatives in dogs. This is most likely due to different metabolites in dog urine compared to humans. 

Clinical suspicion should lead to further questioning of the owner. It can be difficult to approach the subject, but being straightforward and outwardly asking about possible marijuana exposure is the best way. Owners often do not want to confess about the possibility of marijuana due to concern of legal ramifications. Veterinarians are not obligated to report marijuana intoxications to the police, so some owners need to be reassured that the police will not be involved. If owners are still reluctant to divulge any information, discussing additional diagnostics and cost involved to rule out other potential causes of the clinical signs is often enough to get owners to admit to the possibility of marijuana ingestion.

Treatment for marijuana ingestion/toxicity will depend on the severity of clinical signs and the time since exposure/ingestion. As with many toxicities, decontamination is the cornerstone of treatment, including induction of emesis. Emesis induction should only be performed if it is safe to do so. If the animal is too sedate to protect its airway or is non-ambulatory, emesis should not be performed. Activated charcoal also should only be given if safe to do so. Repeated doses of activated charcoal should be given due to the enterohepatic recirculation (every 6-8 hours for the first 24 hours). If it is not safe to induce emesis, other decontamination options should be considered including gastric lavage under anesthesia and enemas. Enemas can be very useful to decontaminate a patient, as it is quick and simple to perform. Activated charcoal retention enemas can also be used in patients unable to receive oral charcoal. In addition to decontamination, IV fluid diuresis will help to eliminate the metabolites of marijuana throughout the kidneys. 

Lipid therapy may also be beneficial in the treatment of marijuana toxicities as it is fat soluble. Monitoring of the patients neurological status heart rate, respiratory rate, and temperature should be performed every 2-4 hours. Patients that are very hyperesthetic/agitated should be kept in a dark quiet area to help reduce external stimuli. Sedation with diazepam may also be useful in these patients.

Prognosis for recovery is excellent. The rate of recovery is dependent of the dose and the route of exposure. Animals with exposure to second hand smoke usually recover within a few hours. Animals ingesting small doses often recover completely in 12-24 hours. Ingestion of large doses may take 24-72 hours.

Brown, A and Mandell, D. Illicit Drugs. Small Animal Critical Care Medicine 2013;(342-345)
Donaldson, C. (2002) Marijuana Exposure in Animals. Vet Med 2002;97: 437-439.
Janczyk, P, Donaldson, C, Gualtney, S. Two Hundred and Thirteen Cases of Marijuana Toxicoses in  Dogs. Veterinary and Human Toxicology 2004; 46 (1): 19-21

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Sarah Attwaters's picture

Working In Vancouver,BC we would see a lot of marijuana toxicity. My current clinic both Dr's on staff have never seen a case before. I came down to the treatment area while their discussing what on earth could be going on. I take one look and go, "oh heck, that dogs stoned!" Needless to say, we spoke with the owner and sure enough. Both vets were happy to diagnosis and treat for marijuana.

Amy Tomasko's picture

Has anyone tried running urine drug screens using feces diluted in sterile water as opposed to urine? Just wondering if this would give a positive result if marijuana was ingested.