In the last decade or so, researchers have published a number of very high-quality randomized trials and quasi-experiments. Of the 27 studies in the new review, 21 have randomized designs. Together, these flip the conclusion.

“These results demonstrate A.A.’s effectiveness in helping people not only initiate but sustain abstinence and remission over the long term,” said the review’s lead author, John F. Kelly, a professor of psychiatry at Harvard Medical School and director of the Recovery Research Institute at Massachusetts General Hospital. “The fact that A.A. is free and so widely available is also good news.

“It’s the closest thing in public health we have to a free lunch.”

Studies generally show that other treatments might result in about 15 percent to 25 percent of people who remain abstinent. With A.A., it’s somewhere between 22 percent and 37 percent (specific findings vary by study). Although A.A. may be better for many people, other approaches can work, too. And, as with any treatment, it doesn’t work perfectly all the time.

Rigorous study of programs like Alcoholics Anonymous is challenging because people self-select into them. Those who do so may be more motivated to abstain from drinking than those who don’t.

Unless a study is carefully designed, its results can be driven by who participates, not by what the program does. Even randomized trials can succumb to bias from self-selection if people assigned to A.A. don’t attend, and if people assigned to the control group do. (It may go without saying, but we’ll say it: It would be unethical to prevent people in a control group from attending Alcoholics Anonymous if they wanted to.)