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The FDA on Tuesday issued a drug safety communication that warned that patients treated with Pfizer’s antibiotic Zyvox (linezolid) in combination with certain antidepressants may be at risk of serotonin syndrome. The agency noted that Zyvox, which is used to treat MRSA and other serious infections, inhibits an enzyme that is involved with the breakdown of serotonin. As such, the FDA said that Zyvox shouldn’t be given to patients who are taking antidepressants and have added instructions for use of the products together to both the Zyvox and antidepressant labels.

Commenting on the news, Pfizer spokeswoman Kristen Neese said the current US package insert for Zyvox "already includes prominent information regarding the potential for serotonergic interactions, the risk of serotonin syndrome and the need for careful observation of patients prescribed Zyvox who are on such agents." She added that the drugmaker hasn’t identified any new safety signals related to those drug interactions. Pfizer recorded sales of nearly $1.2 billion from Zyvox last year.

In a separate drug safety communication also issued Tuesday, the FDA warned of a similar interaction between antidepressants and methylene blue, which is used as a dye and as an emergency treatment of cyanide poisoning.

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