WASHINGTON — Kathy Hornbach of Tucson is not wasting any time before using her new health insurance coverage, which took effect on New Year's Day. Ms. Hornbach, 57, has an appointment with a cardiologist on Thursday for a stress test.

"I've had some heart palpitations, and my mom's side has a history of heart problems starting early," she said Wednesday in a telephone interview. "So it's mostly just to double-check that everything is O.K."

Ms. Hornbach, who has had breast cancer and retired early from the technology industry, said that insurance companies in Arizona had refused to cover her until about two years ago, when she got a policy with monthly premiums of $285 and a deductible of $5,500 a year. Last month, using the federal insurance exchange, she bought a midlevel silver plan with lower premiums and deductible.

"It's a better policy — lower out-of-pocket, more choice of doctors," she said. "This is a very happy day."

Consumers around the country began using coverage provided by the new health care law on Wednesday, the same day that Medicaid expanded to hundreds of thousands of people in about half the states. Many provisions of the 2010 health care law offering new benefits and protections to consumers, including those with pre-existing conditions, also took effect.

Hospitals said they were getting ready for an influx of newly insured patients, but many health care providers said the pace was slower than usual because of the New Year's holiday. In a typical report, Clay Holtzman, a spokesman for Swedish Medical Center in Seattle, said the system's hospitals were not seeing an immediate surge.

"We might at some point down the road, since we have spent a lot of time informing uninsured patients of their options under the exchange and expanded Medicaid," Mr. Holtzman said in an email. "But it depends on if those patients chose plans that include us."

Swedish is one of the largest hospital systems in the region, but Mr. Holtzman said it had been excluded from the networks of providers used by the two largest health plans on the state's insurance exchange.

Some people using their new insurance discovered that they could be responsible for substantial co-payments and other out-of-pocket costs.

Nancy M. Schlichting, the chief executive of the Henry Ford Health System in Detroit, said that one patient who visited the emergency room of the system's flagship hospital on Wednesday tried to fill a prescription and found that the co-payment would be $84 — more than she was accustomed to paying. She got a similar drug with a lower co-payment, illustrating the need for patients to pay close attention to details of their drug coverage, Ms. Schlichting said.

In San Antonio, at a 24-hour Walgreens store, only a few vehicles were lined up at the drive-through window at midday, and no one was waiting in line to pick up prescriptions at the indoor pharmacy counter.

"It's dead," said Leslie Castillo, a pharmacist on duty. "We've had a few regulars come by, but no one has come in today with a new insurance card or wanting us to look up their benefits under Obamacare."

One reason, Ms. Castillo said, was that most doctors were not seeing patients on the New Year's holiday. But she added, "We'll probably be packed tomorrow."

Kenneth E. Raske, the president of the Greater New York Hospital Association, said: "Today is a historic occasion for the health care community. The coverage expansion kicks in for hundreds of thousands of people in New York State and millions across the country, who will enjoy the comfort of knowing they won't have to worry about health care bills if they get sick."

Danny Cottrell, the owner of a pharmacy in Brewton, Ala., said he had helped several people sign up for coverage. One customer, who has $3,500 to $4,000 a year in prescription drug costs, qualified for federal subsidies and chose a plan with a premium of about $300 a month and an annual deductible of $500.

"He will definitely come out ahead," Mr. Cottrell said. "He will save at least $7,500 a year on medical bills."

Dr. Michael W. Cropp, the president of Independent Health, an insurer in Buffalo, said, "I anticipate a lot of uncertainty and confusion and some frustration" as consumers begin to use their new insurance policies.

"The website for the New York exchange is now working for most enrollment purposes," Dr. Cropp said. "But I have concerns about how funds will flow from the federal government to health plans for members receiving federal subsidies."

Expecting a continued battle over health care, the White House moved Wednesday to recruit volunteers for its campaign to defend and promote the law, which is likely to be a defining issue in many congressional races this year. A White House website invites supporters and beneficiaries of the law to provide their names, email addresses and personal experiences.

"Whether you have new coverage today or know someone who does, we want to hear your story," David Simas, an aide to President Obama, said in an email to people who had expressed interest in the issue.

Jessica Santillo, a White House spokeswoman, said the invitation was part of a systematic new effort by the administration to "highlight stories of everyday Americans benefiting from the law."

The administration hopes to encourage enrollment and reverse public opinion polls that show approval of the health care law lagging behind disapproval.