One January morning in 2021, Carol Rosen took a standard treatment for metastatic breast cancer. Three gruesome weeks later, she died in excruciating pain from the very drug meant to prolong her life.

Rosen, a 70-year-old retired schoolteacher, passed her final days in anguish, enduring severe diarrhea and nausea and terrible sores in her mouth that kept her from eating, drinking and eventually speaking. Skin peeled off her body. Her kidneys and liver failed.

“Your body burns from the inside out,” said Rosen’s daughter, Lindsay Murray, of Andover, Massachusetts.

Rosen was one of more than 275,000 cancer patients in the U.S. who are infused each year with fluorouracil, known as 5-FU, or, as in Rosen’s case, take a nearly identical drug in pill form called capecitabine.

These common types of chemotherapy are no picnic for anyone, but for patients who are deficient in an enzyme that metabolizes the drugs, they can be torturous or deadly.