A few days after Donald J. Trump was elected president, I started getting anxious phone calls from some of my patients. They were not just worried about the direction President-elect Trump might take the nation, but about how they were going to fare, given their longstanding and serious mental illnesses.

“Will I still have insurance and have my medications covered?” one depressed patient asked me.

As a psychiatrist, I wish that I could be more reassuring to my patients during a highly stressful political transition, but in truth, they have reason to worry.

Mr. Trump campaigned on a promise to “repeal and replace” the Affordable Care Act, and his pick for secretary of health and human services, Tom Price, is a staunch opponent of Obamacare. The 2010 law provides medical coverage to an estimated 20 million Americans and specifically included mental health and substance abuse treatment as one of 10 “essential benefits” that all private insurers and Medicaid have to cover.

For the 43.6 million American adults living with a psychiatric illness and the 16.3 million who have an alcohol use disorder, it is hard to exaggerate the importance of this. Until this law was passed, people with mental illness and substance abuse problems were subject to capricious annual or lifetime limits on coverage, higher deductibles or no coverage at all.