Cannabinoid hyperemesis syndrome (CHS) is little known amongst clinicians and is characterised by abdominal pain and cyclical vomiting, after intense consumption of cannabis over several years. It does not respond to treatment with antiemetics, but does respond to showers in very hot water. We present the case of a 24-year-old patient whose antecedents included cannabis consumption since the age of 14 and an episode of spontaneous idiopathic pneumomediastinum (PM) the previous month, which presented an association of CHS and secondary pneumomediastinum with the intense, repeated vomiting of the CHS. He was given topical capsaicin and 5 mg of intravenous haloperidol, and was kept for 48 hours in the in A and E Short Stay Unit. This isolated clinical observation appears to indicate the need to rule out CHS as the cause of PM in young patients and, similarly, to consider the presence of PM in the clinical exploration of young people with CHS and in patients with pathologies whose aetiology might be influenced by an increase in intrathoracic pressure.