Victoria Toline would hunch over, steady her hands and draw a bead of liquid from a vial with a dropper. It was a delicate operation — extracting ever tinier doses of the antidepressant she had taken for three years and was desperately trying to quit.

“Basically that’s all I have been doing — dealing with the dizziness, the confusion, the fatigue, all the symptoms of withdrawal,” said Toline, 27, of Gig Harbor, Wash. It took her nine months to stop.

Long-term use of antidepressants is surging in the United States, said an analysis of federal data by the New York Times. Some 15.5 million Americans have been taking the medications for at least five years, double the rate since 2010 and more than triple since 2000.

The drugs have helped millions of people ease depression and anxiety, and are widely regarded as milestones in psychiatric treatment. Many 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 anticipated that a few patients might experience withdrawal symptoms — they called it “discontinuation syndrome.” Yet withdrawal has never been a focus of drugmakers or regulators, who felt antidepressants could not be addictive.

Antidepressants were originally considered for episodic mood problems, to be taken for six to nine months. Then studies suggested that “maintenance therapy,” longer-term use, could sometimes prevent a return of depression, but those trials rarely lasted more than two years. Once a drug is approved, physicians have wide latitude in how they prescribe it.

Most people are put on these drugs in primary care, after a very brief visit,” said Dr. Allen Frances, a professor emeritus of psychiatry at Duke University. “Usually there’s improvement, and often it’s based on the passage of time or placebo effect. But the patient and doctor … give the antidepressant credit it doesn’t deserve.”

The Times analyzed data gathered since 1999 as part of the National Health and Nutrition Examination Survey. Overall, more than 34.4 million adults took antidepressants in 2013-14, up from 13.4 million in the 1999-2000 survey. Adults older than 45, women and whites are more likely to take antidepressants than younger adults, men and minorities. But usage is increasing in older adults across the demographic spectrum.

“What you see is the number of long-term users just piling up year after year,” said Dr. Mark Olfson of Columbia University. Olfson and Dr. Ramin Mojtabai of Johns Hopkins University assisted in the analysis.

Antidepressants commonly cause emotional numbing, sexual problems and weight gain.

Brands with a relatively short half-life, like Effexor and Paxil, appear to cause more withdrawal symptoms more quickly than those that stay in the system longer, like Prozac. “The truth is that the state of the science is absolutely inadequate,” said Dr. Derelie Mangin of McMaster University in Hamilton, Ontario. “We don’t have enough information about what antidepressant withdrawal entails, so we can’t design proper tapering approaches.”

In interviews, dozens of people recounted similar stories: The drugs relieved mood problems, but after a year or so, it wasn’t clear whether it was having any effect. Yet quitting was far harder, and stranger, than expected. “It took me a year to come completely off — a year,” said Dr. Tom Stockmann, 34, a psychiatrist in East London, who experienced lightheadedness, confusion, vertigo and brain zaps, when he stopped taking Cymbalta after 18 months. “I had no idea how hard it would be.”