In any case, the rules about private co-payments, as they are called, in cancer care are contradictory and hard to understand, said Nigel Edwards, the director of policy for the N.H.S. Confederation, which represents hospitals and other health care providers. “I’ve had conflicting advice from different lawyers,” he said, “but it does seem like a violation of natural justice to say that either you don’t get the drug you want, or you have to pay for all your treatment.”

Karol Sikora, a professor of cancer medicine at the Imperial College School of Medicine and one of Dr. Charlson’s co-authors, said that co-payments were particularly prevalent in cancer care. Armed with information from the Internet and patients’ networks, cancer patients are increasingly likely to demand, and pay for, cutting-edge drugs that the health service considers too expensive to be cost-effective.

“You have a population that is informed and consumerist about how it behaves about health care information, and an N.H.S. that can no longer afford to pay for everything for everybody,” he said.

Professor Sikora said oncologists were adept at circumventing the system by, for example, referring patients to other doctors who can provide the private medication separately. As wrenching as it can be to administer more sophisticated drugs to some patients than to others, he said, “if you’re a doctor working in the system, you should let your patients have the treatment they want, if they can afford to pay for it.”

In any case, he said, the health service is riddled with inequities. Some drugs are available in some parts of the country but not in others. Waiting lists for treatment vary wildly from place to place. Some regions spend $280 per capita on cancer care, Professor Sikora said, while others spend just $90.

In Mrs. Hirst’s case, the confusion was compounded by the fact that three other patients at her hospital were already doing what she had been forbidden to do  buying extra drugs to supplement their cancer care. The arrangements had “evolved without anyone questioning whether it was right or wrong,” said Laura Mason, a hospital spokeswoman. Because their treatment began before the Health Department explicitly condemned the practice, they have been allowed to continue.

The rules are confusing. “It’s quite a fine line,” Ms. Mason said. “You can’t have a course of N.H.S. and private treatment at the same time on the same appointment  for instance, if a particular drug has to be administered alongside another drug which is N.H.S.-funded.” But, she said, the health service rules seem to allow patients to receive the drugs during separate hospital visits  the N.H.S. drugs during an N.H.S. appointment, the extra drugs during a private appointment.