A critical drug that serves as the backbone of treatment for most childhood cancers, including leukemias, lymphomas and brain tumors, has become increasingly scarce, and doctors are warning that they may soon be forced to consider rationing doses.

Persistent shortages of certain drugs and medical supplies have plagued the United States for years, but physicians say the loss of this medication, vincristine, is uniquely problematic, as there is no appropriate substitute.

“This is truly a nightmare situation,” said Dr. Yoram Unguru, a pediatric oncologist at the Herman and Walter Samuelson Children’s Hospital at Sinai in Baltimore. “Vincristine is our water. It’s our bread and butter. I can’t think of a disease in childhood cancer that doesn’t use vincristine.”

Shortages of the chemotherapy drug, which is on back order, will likely affect children throughout the country, he said, obligating physicians to make difficult decisions.