If you’re a Trekkie, or even a casual follower of the Star Trek franchise, then you know that (along with his pointed ears) Spock was known for having green blood. If you don’t happen to know what I’m talking about (don’t worry, it just means you don’t live in your mom’s basement), Spock was a member of a fictional race of humanoids called Vulcans. The Vulcans’ home planet was a dry, desert world, so they evolved in a way that gave them copper-based blood. This, in turn, gave it a green color.

As fantastical as the idea of green blood may sound, it is one of many strange but true science facts Star Trek creator Gene Roddenberry used for his writings.

If you are a user of the drug sumatriptan, you actually run the risk of your blood turning green. Sumatriptan—usually under the name-brand Imitrex—is most commonly used to treat migraine headaches. Without getting too technical, the drug works by blocking certain receptors. For most people, there are few side effects. But since sumatriptan contains sulfur, it can do funny things to your blood if you take too much of it.

Such as giving you sulfhemoglobinemia, which is just a nerdy way of saying green blood.

Sulfhemoglobinemia happens when sulfur atoms are integrated with hemoglobin molecules. Sulfide ions then combine with ferric ions to reduce the blood’s ability to carry oxygen, which will turn a person’s blood green and possibly lead to life-threatening consequences. In most known cases, the patient’s condition improved and their blood went back to normal after the red blood cells naturally turned over. But, there is one interesting case from Canada.

In this particular instance, a middle-aged man fell asleep in a kneeling position, which caused “compartment syndrome,” or an insufficient blood flow below his knees. When the doctor drew blood, as is standard, he was shocked to see that it was green! It was later discovered that the gentleman was suffering from sulfhemoglobinemia caused by an overdose of Imitrex.

His day started with a migraine and ended with green blood and surgery on his legs—poor guy.