Oregon has allowed what opponents call “assisted suicide” and supporters term “aid in dying” since 1997, and, after a Supreme Court ruling in 2006 that affirmed the law, Washington, Montana and Vermont have also approved the practice.

Opponents have long raised concerns that ill and disabled people could be coerced into choosing death over more care, which can be expensive and burdensome. The Catholic Church, which considers suicide a sin, also helped lead opposition.

Dr. Aaron Kheriaty, director of the medical ethics program at the University of California, Irvine, School of Medicine, said he expected a number of hospitals would refuse to offer life-ending drugs, to preserve their own reputations and relationships with patients.

“I worry about what this is going to do to the perception of medicine,” he said. “I think you’re going to see more and more mistrust of medical professional by patients worried about what will happen if they enroll in end-of-life care.”

Past efforts to allow doctors to help patients end their lives in California had failed. But this year, support for more end-of-life choices was galvanized around the country by the case of Brittany Maynard, 29, a Bay Area woman who received a diagnosis of terminal brain cancer. Before she died last November, Ms. Maynard became a spokeswoman for the “death with dignity” movement, moving her family to Oregon so she could die on her own terms, and drawing national attention to her cause.