If the finding holds true, it could undercut an argument for the new health care reform law

Like TIME on Facebook for more breaking news and current events from around the globe!

Expanding health insurance coverage for the poor leads to a significant increase in costly emergency room visits, according to a new study. The finding, published Thursday in the journal Science, raises the possibility of trouble ahead as millions of people gain health insurance through expansion of Medicaid under the new health care reform law, coverage that began kicking in on Wednesday.

The 18-month study followed 25,000 low-income Oregonians who won Medicaid coverage in a lottery as part of the Oregon Health Insurance Experiment, a major policy research initiative. Researchers observed a 40 percent increase in emergency room visits among the newly insured.

According to the study’s authors, the correlation between increased emergency room and expanded health coverage was so strong it held true “across a broad range of types of visits, conditions, and subgroups, including increases in visits for conditions that may be most readily treatable in primary care settings.”

If it holds true, the study’s central finding threatens to undermine an argument made by proponents of the Affordable Care Act — including President Barack Obama — that the number of costly emergency room visits would decrease if more people had insurance that covered the cost of seeing a primary care doctor.

“I think that it’s very important that we provide coverage for all people because if everybody’s got coverage, then they’re not going to the emergency room for treatment,” Obama said in 2009, according to the New York Times.

An Obama administration spokesperson said the study’s time frame was too short to see behavioral changes in people accustomed to seeking medical care through the emergency room, and pointed to a longer-term study in Massachusetts, which showed expanded coverage led to an eight-percent decline in emergency room use over several years.

Dr. Amitabh Chandra, director of health policy research at the Harvard Kennedy School of Government, called the study’s results “breathtaking” and said that in previous studies the results were too ambiguous to reach a clear conclusion.

“You would conclude what you wanted to conclude depending on which side of political aisle you were on,” he told the Times. “Now we have an answer.”

Under the Affordable Care Act, nearly 25 million Americans could gain health insurance, roughly half of them through Medicaid.

[Science]