The Vulcan race in the sci-fi TV series Star Trek has green blood, but in all Earth vertebrates it is red. A case report describes a patient who was found during surgery to have a dark green colouration of the blood, subsequently ascribed to a drug he was taking. He presented with compartment syndrome requiring urgent surgery. On presentation he had no obvious unexpected abnormalities apart from a rather dark complexion. Insertion of an arterial catheter was difficult, but when successful this yielded dark, greenish-black blood. Laboratory analysis revealed the presence of sulphaemoglobin. This is formed when a sulphur atom is incorporated into the porphyrin ring of the haeme group of haemoglobin and is not effective as an oxygen transporter. Its formation can be caused by medications including sulphonamides, and the patient’s drug history suggested the source of the problem: he was taking diclofenac, zopiclone and sumatriptan, which he took in a dose of 200mg daily – well in excess of recommendations. Sumatriptan includes a sulphonamide group. The patient recovered uneventfully from surgery and was weaned off sumatriptan after discharge. When seen five weeks later, his blood was normal.