A hospital operating theatre turned into a scene from the X-files when a patient started oozing dark greenish-black blood, it emerged today.

However, the cause of the 42-year-old man's unusual blood colour was not extraterrestrial but traced to the migraine medication he was taking.

The colour of the man's blood returned to normal after he stopped taking the drug, according to the doctors who treated him at St Paul's hospital in Vancouver, Canada.

The patient had been taking large doses of the migraine medication sumatriptan - 200 milligrams a day, which the doctors treating him believed had caused a rare condition called sulfhaemoglobinaemia, where sulphur is incorporated into the oxygen-carrying protein haemoglobin in red blood cells.

Writing in medical journal the Lancet, the doctors wrote: "The patient recovered uneventfully, and stopped taking sumatriptan after discharge. When seen five weeks after his last dose, he was found to have no sulfhaemoglobin in his blood."

The man had been admitted for surgery after developing a dangerous condition in both lower legs after falling asleep in a sitting position.

In the condition, known as "compartment syndrome", swelling and pressure leads to restricted blood flow that causes tissue and nerve damage. Doctors decided to perform an urgent limb-saving procedure, known as fasciotomies, which involves making surgical incisions to relieve pressure and swelling.

Attempts to insert a catheter into one of the patient's arteries prior to the operation yielded the "dark greenish-black blood". A sample was sent for analysis while the surgery went ahead successfully.