The number of people seeking addiction treatment could double under the Affordable Care Act, the Associated Press reports. Under the new law, four million people with drug and alcohol problems will become eligible for insurance coverage. The surge of new patients is likely to strain the substance abuse treatment system, the AP notes.

How many new patients will seek addiction treatment will depend in part on how many states decide to expand their Medicaid programs.

“There is no illness currently being treated that will be more affected by the Affordable Care Act than addiction,” Tom McLellan, CEO of the nonprofit Treatment Research Institute, told the AP. “That’s because we have a system of treatment that was built for a time when they didn’t understand that addiction was an illness.”

The new law designates addiction treatment as an “essential health benefit” for most commercial insurance plans, meaning the plans must cover it.

Substance abuse treatment is to a large extent publicly funded, and run by counselors who have limited medical training, according to the article. Programs are already running over capacity in many places, and have been hit by government budget cuts. The increase in patients could result in long waiting lists, treatment agencies warn.

According to the 2012 National Survey on Drug Use and Health, 23.1 million people ages 12 and older needed treatment for an illicit drug or alcohol use problem last year, but only 2.5 million received treatment at a specialty facility. About one-quarter of those who need treatment but do not receive it lack insurance, according to the article.