Cannabinoid hyperemesis syndrome is characterized by chronic, heavy use of cannabis, recurrent episodes of severe nausea and intractable vomiting, and abdominal pain. Temporary relief of symptoms is achieved by taking a hot bath or shower, and resolution of the problem when cannabis use is stopped. Failure to recognize the syndrome leads to misdiagnoses such as psychogenic vomiting, the cyclic vomiting syndrome, an eating disorder or ‘drug-seeking behaviour’, and may lead to extensive, expensive and unproductive investigations, psychiatric referrals and ineffective treatments. Other than stopping cannabis use, there is no proven treatment. Why a substance known for its antiemetic properties should cause such a syndrome is unknown.