The U.S. Food and Drug Administration has discovered a potentially fatal cocktail of drug interactions when combining any one of a group of popular antidepressants with Pfizer Inc.'s powerful antibiotic Zyvox.

Zyvox, approved by the FDA in 2000, is considered one of the last lines of defense against such "superbugs" as MRSA. But the agency said in a warning to healthcare providers issued late Monday that it has received reports of serious damage to the central nervous system resulting from interactions between Zyvox and such popular antidepressants as GlaxoSmithKline's Paxil, Eli Lilly's Cymbalta and Pfizer's Zoloft.

Pfizer, which is based in New York, has its largest research and development facility worldwide in Groton.

The FDA said the interactions between Zyvox and these psychiatric medications can result in toxic levels of serotonin - known as the "happiness hormone"- building up in the brain.

"This is referred to as Serotonin Syndrome - signs and symptoms include mental changes (confusion, hyperactivity, memory problems), muscle twitching, excessive sweating, shivering or shaking, diarrhea, trouble with coordination, and/or fever" the FDA said in its warning.

The FDA said warning labels will be placed on Zyvox containers as well as on antidepressants identified as being so-called serotonergic psychiatric medications. The 29 medications listed by the FDA include antidepressants Prozac, Effexor, Pristiq, Norpramin, Vivactil, Wellbrutin and Buspar.

Zyvox, according to the FDA, "should generally not be given to patients taking serotonergic drugs. However, there are some conditions that may be life-threatening or require urgent treatment," in which case Zyvox may be required.

The FDA recommended to doctors that patients be temporarily taken off drugs such as Zoloft if they prescribe Zyvox as an antibiotic. Their psychiatric medicine can be restored 24 hours after being off Zyvox, the agency said.

The agency's Adverse Event Reporting System first alerted the FDA to the drug-interaction problem.

"Some deaths were reported," the FDA said. "The literature search identified additional case reports, including at least one case with a fatal outcome."

The FDA said another drug, methylene blue, also appears to exhibit similar interactions with certain psychiatric medicines.

Zyvox, which Pfizer acquired in its 2002 merger with Pharmacia, contributed about $1.2 billion in sales to the company last year. Its main ingredient, linezolid, was a breakthrough medication, the first antibiotic with a new mechanism of action in nearly 40 years.

What patients should do

• You may need to temporarily stop taking your serotonergic psychiatric medication if it becomes necessary to take Zyvox.

• Do not stop taking your serotonergic psychiatric medicine without first talking to a healthcare professional.

• Make sure your healthcare professional knows about all the medications you are taking.

• Contact your healthcare professional immediately if you are taking a serotonergic psychiatric medication and develop any of the following symptoms: mental changes (confusion, hyperactivity, memory problems), muscle twitching, excessive sweating, shivering or shaking, diarrhea, trouble with coordination, and/or fever.

• Report any serious side effects to the FDA MedWatch program using the information in the "Contact Us" box at the bottom of the page.

Source: U.S. Food and Drug Administration