President Obama’s national health care law, the Affordable Care Act, will extend health care coverage to tens of millions of uninsured Americans.

But health care coverage isn’t the same as health care.

The problem is a shortage of doctors. The New York Times reports that, just as the ACA is poised to add millions to the health insurance rolls, the U.S. is on the brink of a critical doctor shortage. The Association of American Medical Colleges estimates that by 2015, the U.S. will have 62,900 fewer doctors than needed, a figure that will more than double by 2025.

Critics fear such shortages will result in long waits for treatment, a problem that plagues universal health care programs. In Britain, the number of patients waiting more than six months for inpatient treatment rose 43 percent in 2011, even as the National Health Service treated fewer patients.

The British Medical Association said the situation was inevitable: “Given the massive financial pressures on the NHS, it was always likely that hospital activity would decrease and waiting times would increase,” said a spokesperson.

Massachusetts, which adopted universal health care in 2006, is having similar problems. The Associated Press reports that a survey last year by the Massachusetts Medical Society found long waits just to get a doctor’s appointment: an average of 48 days for an internist and 36 days for a physician of family medicine — and more than half of primary care doctors are no longer taking new patients. To make matters worse, state lawmakers capped reimbursements for doctors and hospitals in a desperate effort to stem rising health care costs.

Part of the problem is that the baby boom generation is getting older. “Older Americans require significantly more health care,” said Dr. Darrell G. Kirch, president of the Association of American Medical Colleges. “Older individuals are more likely to have multiple chronic conditions, requiring more intensive, coordinated care.”

But the national health reform law itself is creating challenges. The ACA will funnel millions of uninsured into an expanded Medicaid program which is already suffering from a doctor shortage due to low reimbursements.

How will the program handle millions more patients when its current roster of doctors is dwindling?

In addition, about a third of the nation’s doctors are nearing retirement. But the federal law may prompt even more doctors to leave the profession. A survey of physicians by The Doctors Company, the nation’s largest medical liability insurer, found that 43 percent of the respondents were considering retiring within the next five years because of the federal law.

The future may not look much better. Nine out of 10 physicians who responded to the survey said they wouldn’t recommend health care as a profession. One primary care physician commented, “I would not recommend becoming an M.D. to anyone.”

Dr. Donald J. Palmisano, former president of the American Medical Association, warns, “Today, we are perilously close to a true crisis as newly insured Americans enter the health care system and our population continues to age.” If current physicians leave the practice early because of the health law, the problem will get even worse.

Medical schools are gearing up to turn out more doctors, but it will take at least a decade to produce an additional 3,000 doctors, a tiny fraction of what’s needed.

The Affordable Care Act — and our state’s own health reform law — were supposed to increase preventive care and reduce costly emergency room use by the uninsured. But with a shortage of primary care physicians, preventive care will remain elusive, and our emergency rooms will stay jammed for decades.

So, our state and federal officials must answer this question: How will you ensure that people actually get the affordable health care you promised?