A team of Canadian surgeons got a shock when the patient they were operating on began shedding dark greenish-black blood.
In this case, the unusual colour of the 42-year-old’s blood was down to the migraine medication he was taking.
The man’s leg surgery went ahead successfully and his blood returned to normal once he eased off the drug.
The patient had been taking large doses of sumatriptan – 200 milligrams a day.
This had caused a rare condition called sulfhaemoglobinaemia, where sulphur is incorporated into the oxygen-carrying compound haemoglobin in red blood cells.