Cancer doctors across the United States are running short of essential generic chemotherapy drugs, and some fear the squeeze could force widespread rationing, The New York Times reported.

The shortages trace to manufacturing problems, shipping delays and decisions by some companies to stop making the medicines, according to the U.S. Food and Drug Administration (FDA).

The decades-old infusions are hard to produce in sterile plants and command very low prices. Yet, they remain among the most effective options for cancers of the breast, lung and head and neck when more targeted treatments aren't available, The Times reported.

For ifosfamide, used to treat sarcomas, lymphomas and testicular cancers, some doctors are already meeting to decide who gets treated.

"There are some individuals who won't be able to receive this drug, even if it is the best option for them, which puts pharmacists, doctors, patients and families in a horrific situation," said Dr. Andrew Shuman, a head and neck cancer surgeon and professor at the University of Michigan.