The plant Cannabis sativa has been used for centuries for the effects of its psychoactive resins. The term "marijuana" typically refers to tobacco-like preparations of the leaves and flowers. The plant contains more than 400 chemicals but the cannabinoid δ-9-tetrahydrocannabinol (THC) is the major psychoactive constituent. "Hashish" is the resin extracted from the tops of flowering plants and generally has a much higher THC concentration. Marijuana is the most commonly used illicit drug in the United States. Currently, several states have passed legislation to decriminalize possession of small amounts of marijuana for both medical and personal use and several other states have similar legislation under consideration. The most common form of marijuana use in humans is inhalation of the smoke of marijuana cigarettes, followed by ingestion. In animals, although secondhand smoke inhalation is possible, the most common source of exposure is through ingestion of the owner's marijuana supply. The minimum lethal oral dose for dogs for THC is more than 3 g/kg. Although the drug has a high margin of safety, deaths have been seen after ingestion of food products containing the more concentrated medical-grade THC butter. There are two specific cannabinoid receptors in humans and dogs, CB1 (primarily in central nervous system) and CB2 (peripheral tissues). In animals, following oral ingestion, clinical effects begin within 60 minutes. All of the neuropharmacologic mechanisms by which cannabinoids produce psychoactive effects have not been identified. However, CB1 activity is believed to be responsible for the majority of cannabinoid clinical effects. Highly lipid soluble, THC is distributed in fat, liver, brain, and renal tissue. Fifteen percent of THC is excreted into the urine and the rest is eliminated in the feces through biliary excretion. Clinical signs of canine intoxication include depression, hypersalivation, mydriasis, hypermetria, vomiting, urinary incontinence, tremors, hypothermia, and bradycardia. Higher dosages may additionally cause nystagmus, agitation, tachypnea, tachycardia, ataxia, hyperexcitability, and seizures. Treatment of marijuana ingestion in animals is largely supportive. Vital signs including temperature and heart rate and rhythm must be continually monitored. Stomach content and urine can be tested for cannabinoids. Gas chromatography and mass spectrometry can be utilized for THC detection but usually may take several days and are not practical for initiation of therapy. Human urine drug-screening tests can be unreliable for confirmation of marijuana toxicosis in dogs owing to the interference of a large number of the metabolites in canine urine. False negatives may also arise if testing occurs too recently following THC ingestion. Thus, the use of human urine drug-screening tests in dogs remains controversial. No specific antidote presently exists for THC poisoning. Sedation with benzodiazepines may be necessary if dogs are severely agitated. Intravenous fluids may be employed to counter prolonged vomiting and to help control body temperature. Recently, the use of intralipid therapy to bind the highly lipophilic THC has been utilized to help reduce clinical signs. The majority of dogs experiencing intoxication after marijuana ingestion recover completely without sequellae. Differential diagnoses of canine THC toxicosis include human pharmaceuticals with central nervous system stimulatory effects, drugs with central nervous system depressant effects, macrolide parasiticides, xylitol, and hallucinogenic mushrooms.
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