This retrospective case series describes a novel and unexpected source for marijuana toxicosis in dogs; suspected ingestion of human faeces containing Δ 9 -tetrahydrocannabinol (THC). Medical records from four, 24-h veterinary emergency hospitals in Melbourne, Australia, were reviewed and 15 dogs met the criteria for inclusion in this case series. Clinical signs of marijuana toxicosis included ataxia (n = 13), mydriasis (n = 6), hyperaesthesia (n = 5), urinary incontinence (n = 4) and stupor (n = 3). A urine drug screening test was performed for eight dogs and all were positive for THC. Confirmation of ingestion of human faeces was based on owner-witnessed ingestion (n = 7) or the presence of faecal material within vomit (n = 8). Sites of human faecal exposure were recorded to be a local park (n = 10), beach (n = 1), camp site (n = 1) and walking trail (n = 1). Time from exposure to development of clinical signs ranged between 3 and 6 h (n = 4). All dogs survived to discharge. Ingestion of human faeces containing THC may lead to marijuana toxicosis in dogs. Veterinary staff and owners should be attentive in regard to using appropriate hygiene measures when managing these dogs.