The Food and Drug Administration yesterday approved the strongest possible warning for antidepressants to alert doctors and patients that the drugs could slightly increase the risk of suicidal thinking and behavior in adults 18 to 24.

The so-called black-box warning extends a similar labeling change approved in 2004 for children and adolescents.

An FDA advisory committee recommended the warnings in December after finding a small increase in suicidal thoughts and behavior among young adults taking 11 commonly used antidepressants. The agency said five of every 1,000 patients age 18 to 24 were at higher risk.

"The risk is small, but the risk is real, and it is something that needs to be paid attention to," Thomas Laughren, director of the FDA's division of psychiatry products, said during a news conference.

The FDA's action requires drug companies to submit proposed labeling and updated medication guides within 30 days. A total of 36 drugs - including top-selling Effexor, Zoloft, Lexapro and Cymbalta - would display the warning.

Americans spent $13.5 billion on antidepressants last year, according to data compiled by IMS Health. An estimated 19 million Americans have depression and 16 million are treated with antidepressant drugs. People under 25 account for about 8 percent of the prescriptions.

People currently taking prescription antidepressants should continue to use them and consult their doctors if they have concerns, the FDA said yesterday.

In an unusual move, Laughren said the new labels would carry both a warning and positive information on the benefits of antidepressants. The labels would emphasize weighing the risks associated with the drugs against the danger of failing to treat depression and other psychiatric disorders that contribute to suicide.

Some psychiatrists and advocacy groups had been concerned that a black box might discourage young adults who need help from taking the drugs, which many consider among the most effective treatments for depression.

In 2004, suicides rose 8 percent among 10-to-14-year-olds as antidepressant-prescribing fell in advance of the FDA warning about suicidal thoughts and behavior in children and adolescents, according to government statistics. That same year, suicides among 15-to-19-year-olds rose 12 percent.

Carolyn Robinowitz, president-elect of the American Psychiatric Association, said it was unprecedented for the FDA to include a positive information in its black-box warning. She said the new labeling would provide a more balanced message.

The new labels would also state that antidepressants reduced the risk of suicidal thoughts and behaviors in adults 65 and older, the opposite of what was seen in young adults, the FDA said.

Drugmakers contacted yesterday said they would comply with the FDA's request.

"We are encouraged that the agency has accompanied the warnings with balanced information about the importance of being treated for depression," said Tammy Hull, a spokeswoman for Indianapolis-based Eli Lilly & Co., maker of Cymbalta.

"This step will help ensure that the millions of people with depression who are young adults age 24 and under and their families can make informed treatment decisions."