An 18-year-old female patient presented to the emergency department with non-specific neurological and gastrointestinal symptoms and was found to be pancytopenic. Her vitamin B12 level was low with a normal mean corpuscular volume and her full blood count 2 months previously had been within normal range. She reported heavy use of nitrous oxide over the previous 2 weeks and other investigations revealed no cause for her pancytopenia. Her pancytopenia resolved with discontinuation of nitrous oxide and vitamin B12 treatment. Heavy use of nitrous oxide should be considered as a cause of pancytopenia.