Marijuana & Veterinary Medicine

Suzanne Smither

Tina Wismer, DVM, MS, DABVT, DABT, ASPCA Animal Poison Control Center, Champaign, Illinois

Heather Prendergast, RVT, CVPM, SPHR, Patterson Veterinary University, Las Cruces, New Mexico

ArticleLast Updated June 201611 min readPeer Reviewed
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Marijuana, derived from the cannabis plant, has long been used in human medicine, but its effects on companion animals have only been noted since the mid-to-late 1900s.

When medical marijuana first became legal for human use in California in 1996,1 it was only a matter of time before human patients, pleased with the results of therapeutic cannabis, began giving it to pets with similar conditions.


Suzanne Smither, Fort Lauderdale, Florida

A 2012 study evaluating 125 client-owned dogs in Colorado presented for known or suspected marijuana toxicosis2 found a significant correlation between the number of people registered for medical marijuana licenses (which had increased 146-fold in the previous 5 years) and marijuana toxicosis cases in animals (which had increased 4-fold), although the study reported only 2 deaths from the “ingestion of baked goods made with medical grade tetrahydrocannabinol butter.”

Marijuana & Hemp

The term Cannabis sativa—the genus and species that comprise subspecies of both marijuana and hemp—is often incorrectly used, so distinguishing between the 2 plants, as well as an intermediate variety, can be difficult. (See Definitions, in Marijuana Toxicity section.) Each plant has many chemical constituents known as cannabinoids, including delta-9-tetrahydrocannabinol (THC), which is psychoactive, and cannabidiol (CBD), which is not.3 

The cannabinoid THC is the most recognized because it is the principal psychoactive component of the plant, but it is one of many, according to the US Pharmacopeia: “The potential for therapeutic activity of cannabis is not limited to cannabinoids; emerging research suggests that other minor compounds … also play a role in the complex pharmacology of this botanical.”3


The roots of medicinal cannabis have been entwined with human history for millennia, perhaps starting with the Chinese emperor Fu Hsi around 2900 BC.4

Its use next can be traced from the 9th- and 10th-century Arab world, to Europe and Asia in the Middle Ages, to America, where it arrived with early settlers of Jamestown, Virginia, in 1611.4 

Marijuana joined mainstream medicine in the West in the 1840s, when French psychiatrist Jacques-Joseph Moreau discovered it “suppressed headaches, increased appetites, and aided people to sleep.”4

In America, marijuana was added to the US Pharmacopeia in 1850.4

“We are walking a tightrope.”—Tina Wismer

Legal Restrictions

Restrictions on cannabis use began in the early 20th century, with prohibitionists similar to those who later outlawed alcohol. Massachusetts made marijuana illegal in 1911; 10 other states followed suit from 1915 through 1927. When marijuana was removed from the US Pharmacopeia in 1942, it lost what the American Medical Association called its “remaining mantle of therapeutic legitimacy.”4 In 1970, the Controlled Substances Act classified “marijuana (cannabis)” as a drug with “no accepted medical use.” Still today it is listed in Schedule I along with heroin, peyote, LSD, and Ecstasy.5

Even veterinarians enthusiastic about the benefits they believe these products provide their patients are cautious when discussing them with clients because of legal risks and undetermined therapeutic and toxic dosages. Federal law prohibits veterinarians from prescribing marijuana, even in the 24 states and the District of Columbia where it is currently legal for human medicinal use.6 Hemp-derived supplements are legally available from pet product retailers but are not FDA-regulated. 


Clearly, more research is needed, but obstacles stand in the way. (See Research: Needed but Complicated). Veterinary professionals need to know what is legal and illegal in their state and make the legal status clear to clients when discussing marijuana’s possible benefits for patients.


Therapeutic use of cannabis in pets is a hot topic.

Here are some veterinarians’ views:

Robert Silver, DVM, MS, CVA“There is a huge body of evidence from well-performed research studies showing … the medicinal value of the whole marijuana plant.”

Narda Robinson, DVM, DO, MS, DABMA, FAAMA“THC and cannabis in general have anti-inflammatory effects,” she said. According to a survey she conducted, possible therapeutic uses of cannabis also include easing pain, alleviating separation anxiety and seizures, and stimulating the appetites of cancer patients undergoing chemotherapy. 

Justine Lee, DVM, DACVECC, DABT“I don’t underestimate the therapeutic effects of marijuana. We as veterinarians need to be very cautious because we don’t know the toxic dose; we don’t know the therapeutic dose; and we don’t know the variable amounts of cannabinoids within different products, both hemp and marijuana.”

Tina Wismer, DVM, DABVT, DABT“When animals eat marijuana, they are going to be symptomatic for a much longer time than humans … 24 to 72 hours. It’s all about the dose.”

“If people have (cannabis) products in their homes, they need to make sure to keep them away from children and pets because they can’t control the dosage.”

Marijuana Toxicity

Tina Wismer, DVM, DABVT, DABT, ASPCA, Urbana, Illinois

Clinical Case Summary

Lucy, a 27 lb, 4-year-old spayed beagle, presented with acute-onset lethargy, ataxia, and urinary incontinence. She had vomited once earlier that day, but that was not unusual because, her owners said, she “eats everything.” On physical examination, Lucy was mildly disoriented and hyper-reactive to stimuli (ie, she startled easily, overreacted to hand movements) but was dull when unstimulated and fell asleep. The remainder of her physical examination was within normal limits.

Because Lucy had been unsupervised for several hours before onset of clinical signs, and given her abnormal neurologic status, her owners were asked whether any neurologic toxins (eg, ethanol, ethylene glycol, ivermectin, marijuana, medications [antidepressants, benzodiazepines, barbiturates, opioids, phenothiazines]) were present in the home. The clients told the veterinary team Lucy may have had access to medical marijuana. 


Humans use marijuana (Cannabis sativa) medicinally as an antiemetic and appetite stimulant, and for glaucoma to decrease intraocular pressure, as well as recreationally.1 However, even though the plant has been legalized in some states for medicinal or recreational use, marijuana is still a Schedule I controlled substance under the US Controlled Substances Act.2 (See Definitions.)

Marijuana’s toxicity mostly comes from a resin called delta-9-tetrahydrocannabinol (THC), but the plant also contains other cannabinoids and cannabinols. 

Ingesting marijuana plant material intended for human consumption via inhalation used to be the most common pet exposure, but now that marijuana has been legalized, it is increasingly available in various forms, including dried plant leaves, edibles (eg, cookies, brownies), butters, and concentrates (eg, oils, waxes). Many edibles include chocolate, which can lead to concurrent methylxanthine toxicosis. Selective breeding of marijuana plants has favored increased THC concentrations—the University of Mississippi Potency Monitoring Project reported that THC levels have more than doubled over the last 25 years.3 THC concentrations vary: for example, plant materials contain 1% to 8%, extracts contain 28%, and hash oil contains up to 50% THC.4

The most common clinical signs following marijuana ingestion are lethargy, depression, ataxia, and urinary incontinence—the latter is unique to dogs and caused by THC metabolites that are not produced during human metabolism. Approximately 25% of patients may display hyperexcitability rather than dull mentation. Other common signs include disorientation, hyperesthesia, tremors, bradycardia, hypothermia, and mydriasis. Patients ingesting high THC doses in concentrates or butter products may display more severe neurologic and cardiovascular signs, including coma and hypotension. Clinical signs may be seen 30 minutes following ingestion and last up to 72 hours.

Diagnosis is typically made based on history of possible exposure and clinical signs. A human urine drug test can be used, but it has not been validated in dogs. False negative results are common in dogs, which produce different metabolites than humans.5 When evaluating these patients, diagnostic tests are essential to rule out ethylene glycol toxicosis because antifreeze poisoning in its initial stages can appear clinically similar to marijuana toxicity. Failure to diagnose ethylene glycol toxicosis routinely leads to fatal acute kidney injury.


Marijuana toxicosis is rarely fatal because the drug has a wide safety margin, but patients’ cardiovascular and neurologic status, respiratory function, and body temperature should be closely monitored. Only 2 deaths have been reported to date.6

Marijuana is an antiemetic so inducing emesis likely will be unsuccessful but may be attempted using apomorphine within 30 minutes of oral exposure if the animal is asymptomatic. However, apomorphine does not always work because THC affects the vomiting center; hydrogen peroxide, a local irritant, may work best. Also, THC is highly lipophilic and enterohepatically recirculated; therefore, repeated activated charcoal dosing is sometimes used to decrease the half-life of THC in the body7 but should be reserved only for life-threatening situations.

Recovery from marijuana toxicosis may take 24 hours to several days, depending on the amount ingested.

Patients with few symptoms likely will recover with minimal supportive care, but intravenous lipid emulsion should be used in more symptomatic patients to hasten THC elimination, although recovery may be variable.  The dosing regime is a 1.5 ml/kg initial bolus over 20 to 30 minutes followed by a CRI of 0.25 ml/kg/min for 30 to 60 minutes.8 If the response is inadequate, the 1-hour CRI can be repeated once in 4 hours provided there is no lipemia.

Intravenous fluid administration can be used to correct hypotension or dehydration as warranted. Benzodiazepines or low-dose acepromazine are appropriate sedatives for agitated patients if hypotension is absent. Patients benefit from maintaining normothermia, minimizing sensory stimuli, and providing recumbent nursing care if nonambulatory. 

Recovery may take 24 hours to several days, depending on the amount ingested.7


Lucy was prescribed IV fluids and monitored overnight for declining neurologic status. She experienced mild hypotension that a fluid bolus easily reversed and was discharged the following afternoon with full resolution of clinical signs. 

Marijuana toxicosis is being seen more frequently in veterinary medicine,6 with more than 700* marijuana-related calls to the ASPCA Animal Poison Control Center in 2015. Whether this is due to an actual increase in cases, owners’ changing attitudes making them more willing to seek veterinary care, or owners seeking medical attention because their pets are ingesting more potent forms of marijuana is not known.


Obtaining an accurate history may be difficult when a patient is suspected of illicit drug exposure. (See Questions for a Helpful History.) However, it also is important, so use direct, nonthreatening questions when talking to clients.

*From the author, the ASPCA Animal Poison Control Center medical director.

Team Drug Policy

Heather Prendergast, RVT, CVPM, Synergie, Las Cruces, New Mexico

As veterinarians face the prospect of using cannabis or cannabis components in their patient treatment protocols, practice managers need to have a plan in place to deal with team member use of the substance. 

Some states have legalized the Class 1 Controlled Substance, some allow its use for medical purposes only, and others still have not made the drug legal. However, every practice, no matter its location or number of team members, should have a written drug and alcohol policy. (See Resource.)

Safety is Key

Every veterinary employer must provide a safe work environment for team members, clients, and patients, and a policy addressing drug and alcohol impairment will help ensure the workplace is safe. For example, if a team member is taking a prescribed controlled substance but cannot perform his or her job safely and efficiently, the practice is protected so long as a drug policy is in place. (See Drug Policy Example.)

Different States

Most importantly, be familiar with the varying state laws. According to the Drug Enforcement Agency, marijuana is classified as a Class 1 Controlled Substance and is illegal in all states. However, some states have passed laws that contradict the federal classification and legalize the substance for medical and/or recreational use. A clearly defined practice drug policy is imperative.

In states where marijuana is legalized, the practice policy should state that alcohol and marijuana use will be treated the same way: It is unacceptable to come to work under the influence of either. Alcohol is also legal, but it is illegal to drive under the influence.  

In states where medical marijuana use is approved, a team member does not have automatic permission to be actively under the influence at work. However, accommodations under the Americans with Disabilities Act may be necessary depending on the nature of the job and the safety required.1

Also, the federal Occupational Safety and Health Administration is likely to step in if a marijuana incident occurs in the practice, and the federal illegal status of the drug will likely trump state law.2

Implementing a Policy

A drug testing policy should be in writing and include the testing reason (eg, preemployment, random, postincident) and the consequences of a positive test. Be sure to:

  • Fully explain the policy to the team.

  • Apply the policy consistently to all team members to avoid discrimination charges.

  • Document all testing, including the selection method. (See Drug Testing Recommendations.)

If the practice chooses to test team members randomly, attorneys suggest limiting testing to situations with cause3 (ie, when the manager has reasonable suspicion of drug use), or when the team member has been involved in a workplace accident (eg, tripping, slipping, falling; back injury from carrying heavy patients or products; injury from a patient [dog or cat bite, horse or cow kick]).

Be sure to document the facts—not gossip or rumors—supporting the cause if the practice does decide to test a team member for cause.

If a Drug Free Workplace Policy is implemented, give team members 30 days’ notice and then test every member to avoid discrimination claims. If management suddenly tests 1 team member without a drug testing policy in place, the practice could potentially be sued.


Every veterinary practice must institute a drug and alcohol policy and test team members appropriately and be prepared to discipline or terminate those who screen positive for current use of illegal drugs.1