The survey of 1,144 doctors around the country is the first major look at how physicians' religious or moral beliefs might affect patients' care.
The study, conducted by University of Chicago researchers, found 86 percent of those responding believe doctors are obligated to present all treatment options, and 71 percent believe they must refer patients to another doctor for treatments they oppose. Slightly more than half the rest said they had no such obligation; the others were undecided.
"That means that there are a lot of physicians out there who are not, in fact, doing the right thing," said David Magnus, the director of Stanford University's Center for Biomedical Ethics.
According to an American Medical Association policy statement, doctors can decline to give a treatment sought by an individual that is "incompatible with the physician's personal, religious or moral beliefs."
But the physician should try to ensure the patient has "access to adequate health care."
The survey did not examine whether these doctors act on their beliefs - that is, whether they actually withhold information or refuse to refer patients. But the researchers calculated that tens of millions of Americans might be going to such doctors.
The study was published in today's New England Journal of Medicine and led by Dr. Farr Curlin, a University of Chicago ethicist and internist. The findings were based on a survey sent to 1,820 practicing U.S. family doctors and specialists, chosen randomly from a national database; 63 percent responded.
Doctors describing themselves as very religious, particularly Protestants and Catholics, were much less likely than others to feel obligated to tell patients about controversial treatments or refer them to other doctors, and were far more likely to tell patients if they had moral objections. Overall, 52 percent said they oppose abortion, 42 percent opposed prescribing birth control to 14- to 16-year-olds without parental approval, and 17 percent objected to sedating patients near death.
Female doctors were much more likely than male ones to feel obligated to refer patients for treatments they oppose, far less likely to present their own objections to a patient and slightly more likely to disclose all treatment options.
Dr. Curlin said that in light of the new findings, if a patient "anticipates wanting a controversial treatment and they don't know already if their physician opposes it, then they should ask."