Is Flagyl (metronidazole) a sulfa antibiotic?

From the Guidelines

No, Flagyl (metronidazole) is not a sulfa antibiotic. Metronidazole belongs to a class of antibiotics called nitroimidazoles, which work by disrupting bacterial DNA, particularly in anaerobic bacteria and certain parasites. Sulfa antibiotics, on the other hand, are sulfonamide-containing medications like sulfamethoxazole (found in Bactrim or Septra) that work by inhibiting bacterial folate synthesis. This distinction is important for patients with sulfa allergies, as they can typically take metronidazole safely even if they have a documented sulfa allergy.

Key Points

• Metronidazole is commonly used to treat infections caused by anaerobic bacteria and certain parasites like Giardia, Trichomonas, and Entamoeba.
• The medication comes in oral, intravenous, and topical formulations, with typical adult oral dosing ranging from 250-500 mg three times daily for 7-14 days, depending on the infection being treated 1.
• According to the 2015 IDSA clinical practice guidelines, metronidazole can be used to treat native vertebral osteomyelitis due to Bacteroides species and other susceptible anaerobes, with a recommended dose of 500 mg PO tid to qid 1.
• It's essential to note that sulfa antibiotics, such as trimethoprim-sulfamethoxazole (TMP-SMX), have a different mechanism of action and are used to treat different types of infections, and may not be suitable for patients with sulfa allergies 1.

From the FDA Drug Label

Metronidazole, a nitroimidazole, exerts antibacterial effects in an anaerobic environment against most obligate anaerobes The FDA drug label does not answer the question.

From the Research

Classification of Flagyl

• Flagyl, also known as metronidazole, is a 5-nitroimidazole antibiotic 2, 3, 4, 5.
• It is not a sulfa antibiotic, as sulfa antibiotics are a different class of antimicrobials that work by inhibiting DNA synthesis 5.
• Metronidazole is active against anaerobic bacteria and protozoa, and is commonly used to treat infections such as trichomoniasis, amebiasis, and giardiasis 2, 3, 4.

Mechanism of Action

• Metronidazole works by entering the cells of anaerobic microorganisms and damaging their DNA, ultimately leading to cell death 2, 3.
• It is bactericidal, meaning it kills bacteria directly, rather than simply inhibiting their growth 2, 3.

Spectrum of Activity

• Metronidazole is effective against a wide range of anaerobic bacteria, including Bacteroides fragilis, Clostridium perfringens, and Peptostreptococci 2, 3, 4.
• It is also active against certain protozoa, such as Trichomonas vaginalis, Entamoeba histolytica, and Giardia lamblia 2, 3, 4.