WASHINGTON —  The antidepressant Paxil may raise the risk of suicidal behavior in young adults, its manufacturer GlaxoSmithKline and the U.S. Food and Drug Administration warned Friday in a letter to doctors.

The warning letter was accompanied by changes to the labeling of both Paxil and Paxil CR, a controlled-release version of the drug, also called paroxetine.

A recent analysis of clinical trial data on nearly 15,000 patients treated with both Paxil and dummy pills revealed a higher frequency of suicidal behavior in young adults treated with the drug, according to the letter.

The FDA reported that there were 11 suicide attempts — none resulting in death — among the patients given Paxil in the trials. Just one of the dummy pill patients attempted suicide.

Given that small number, the results "should be interpreted with caution," the FDA said. Eight of the 11 attempts were made by patients between the ages of 18 and 30. All trial patients suffered from psychiatric disorders, including major depression.

A GlaxoSmithKline spokeswoman did not immediately return a message seeking comment. However, in the letter to doctors, Dr. John E. Kraus, the company's director of clinical development for clinical psychiatry in North America, said GlaxoSmithKline continues to believe the drug's benefits outweigh its risks.

The FDA stressed that all patients, especially young adults and those who are improving, should be carefully monitored when treated with Paxil.

In 2004, the FDA ordered strong warnings about the pediatric risk of suicidal tendencies put on antidepressant labels, and began analyzing whether adults face a similar risk.

All antidepressants now carry warnings on their labels cautioning patients and doctors of the risk of suicidal behavior.