A 25-year-old woman showed up at an emergency room in Providence, R.I., with fatigue, shortness of breath, and—most concerning—an obvious blueish hue.

The skin discoloration suggested that her body wasn’t getting enough oxygen. When doctors drew some of her blood, it appeared as a dark, chocolate-brown color rather than the usual vibrant red.

The doctors figured out what had happened after the woman noted that she had had a toothache the night before—and had used a common over-the-counter anesthetic to ease her pain.