The drugs have helped millions of people ease depression and anxiety, and are widely regarded as milestones in psychiatric treatment. Many, perhaps most, people stop the medications without significant trouble. But the rise in longtime use is also the result of an unanticipated and growing problem: Many who try to quit say they cannot because of withdrawal symptoms they were never warned about.

Some scientists long ago anticipated that a few patients might experience withdrawal symptoms if they tried to stop — they called it “discontinuation syndrome.” Yet withdrawal has never been a focus of drug makers or government regulators, who felt antidepressants could not be addictive and did far more good than harm.

The drugs initially were approved for short-term use, following studies typically lasting about two months. Even today, there is little data about their effects on people taking them for years, although there are now millions of such users.