In March of this year, strange reports began to emerge: Synthetic pot was making people’s eyes bleed. And what started out as a handful of people who were affected has grown to more than 250 people across the U.S., with cases continuing to pop up.

Soon after the outbreak began, the culprit was identified as synthetic marijuana contaminated with brodifacoum. Used as a rat poison, brodifacoum can cause severe thinning of the blood, leading to bleeding in multiple parts of the body.

To reverse the vitamin K-blocking effects of warfarin or brodifacoum, people need to take vitamin K supplements. But the amount of vitamin K required to treat brodifacoum poisoning is much higher than that needed during warfarin therapy — for a typical patient on warfarin, doctors might prescribe a single dose of 5 to 10 milligrams of vitamin K, but patients who used the tainted drugs needed as much as 150 mg per day. What’s more, patients with brodifacoum poisoning often needed treatment for many months.

“The sheer volume of vitamin K needed, and the length of time needed to…get [the brodifacoum] fully out of your system,” was surprising, Kelkar said.