May 15 (Bloomberg) -- Migraine headache medicines, including GlaxoSmithKline Plc's Imitrex and Merck & Co.'s Maxalt, caused potentially fatal reactions in at least 11 people, according to today's New England Journal of Medicine.

People using an older class of migraine drugs called triptans may develop serotonin syndrome, which can cause rapid heartbeat, fever, vomiting and shock, according to the first report documenting the syndrome in people who weren't also taking antidepressants.

The U.S. Food and Drug Administration warned doctors and patients in 2006 that combining triptans with antidepressants, including Prozac, Lexapro, Cymbalta and Effexor, could cause too much of the chemical serotonin to accumulate in the brain, possibly leading to serotonin syndrome. The new study is the first to use the FDA Adverse Event Reporting System to focus on triptan drugs alone, said author Offie Soldin, an associate professor at Georgetown University Medical Center in Washington.

``This is a voluntary database so the 11 cases are the ones that have been reported, but there are lots of cases out there that we may not know anything about,'' Soldin said in a telephone interview on May 12.

About 17 percent of women and 6 percent of men get migraines, and almost 4 million prescriptions for triptans are written annually, Soldin said.

FDA Database

A government grant from the National Institute of Child Health and Human Development allowed Soldin to search the FDA database, where she found 38 cases of serotonin syndrome as of a year ago. Of those, 27 were in patients taking the headache drugs and antidepressants and 11 in people taking headache drugs alone, she said. None of the individuals who had the reaction took overdoses of drugs, according to the author.

In the 11 reports of serotonin syndrome, five patients were hospitalized and two cases were classified as life threatening, the study said.

Drugmakers added warnings to prescribing information in April 2007 about the potential for serotonin syndrome with triptans alone, said Holly Russell, a spokeswoman for London- based Glaxo, in an e-mailed statement.

Merck knows of no cases of serotonin syndrome associated just with its Maxalt drug, said Ian R. McConnell, a spokesman for the Whitehouse Station, New Jersey, company.

Triptans relieve migraine pain by narrowing blood vessels and balancing brain chemicals called neurotransmitters. Some drugs for depression also increase serotonin in the brain. The chemical, produced naturally by nerve cells, relays signals and helps regulate mood, sleep, body temperature and blood flow.

To contact the reporter on this story: Catherine Larkin in Washington at clarkin4@bloomberg.net.

Last Updated: May 15, 2008 00:01 EDT