Bactrim vs. Flagyl
See how Bactrim and Flagyl compare on approved uses, dosing, side effects, and cost.
Key Takeaways
Bactrim treats infections like urinary tract infections and pneumocystis pneumonia, while Flagyl targets anaerobic bacteria and parasites such as trichomoniasis and amebiasis.
Both drugs can cause nausea and loss of appetite, but Flagyl carries a boxed warning for carcinogenicity in animal studies and Bactrim carries a warning for severe skin reactions.
Flagyl requires strict alcohol avoidance during and after treatment, while Bactrim is contraindicated in G6PD deficiency and in infants under two months old.
Bactrim vs. Flagyl Drug Summary
Bactrim
Bactrim is a combination sulfonamide antibiotic that pairs sulfamethoxazole and trimethoprim to block two sequential steps of bacterial folate production. It treats infections like urinary tract infections, pneumocystis pneumonia, and shigellosis. It comes as an oral tablet, liquid suspension, or IV infusion, usually taken every 12 hours.
Flagyl
Flagyl, known generically as metronidazole, is a nitroimidazole antimicrobial that treats anaerobic bacterial infections and parasitic infections like trichomoniasis and amebiasis. It is taken as an oral tablet, capsule, or IV infusion, with the dose and length of treatment varying by infection. Brand-name tablets have been discontinued, but generic metronidazole remains widely used.
Bactrim vs. Flagyl Side Effects
Bactrim
Flagyl
Bactrim vs. Flagyl Dosage
Bactrim
Flagyl
| STAGE | DOSE | WHEN |
| Starting dose | 800 mg/160 mg every 12h | At the start |
| Maximum dose | 8-10 mg/kg/day div q6-12h | If needed |
| Dose changes | 3 increases | As tolerated |
| STAGE | DOSE | WHEN |
| Starting dose | 250 mg three times daily | Days 1-7 |
| Maximum dose | 7.5 mg/kg every 6 hours | Days 1-7 |
| Dose changes | 3 increases | As tolerated |
Bactrim
| STAGE | DOSE | WHEN |
| Starting dose | 800 mg/160 mg every 12h | At the start |
| Maximum dose | 8-10 mg/kg/day div q6-12h | If needed |
| Dose changes | 3 increases | As tolerated |
Flagyl
| STAGE | DOSE | WHEN |
| Starting dose | 250 mg three times daily | Days 1-7 |
| Maximum dose | 7.5 mg/kg every 6 hours | Days 1-7 |
| Dose changes | 3 increases | As tolerated |
Bactrim vs. Flagyl Indications
Bactrim
Bactrim is FDA-approved for a range of bacterial infections, including urinary tract infections, pneumocystis pneumonia, and shigellosis. It is also used off-label for skin infections caused by MRSA. It is not FDA-approved for strep throat.
Urinary tract infections
Pneumocystis jirovecii pneumonia
Shigellosis
Acute otitis media in children
Flagyl
Flagyl is FDA-approved to treat trichomoniasis, amebiasis, and infections caused by anaerobic bacteria. It is also used off-label, in a specific dosing regimen, for bacterial vaginosis. It is not effective against viral infections like colds or flu.
Trichomoniasis (symptomatic and asymptomatic)
Amebiasis
Anaerobic bacterial infections
Treatment of partners of trichomoniasis patients
Bactrim vs. Flagyl Pros and Cons
Bactrim
Bactrim has been used safely for decades and remains one of the most affordable antibiotics available. It covers a wide range of common bacterial infections, including some drug-resistant staph infections. It does carry meaningful risks for patients with sulfa allergies or G6PD deficiency.
Pros
Inexpensive, widely available generic
Available as tablet, liquid, and IV form
Effective off-label against MRSA skin infections
No alcohol restriction during treatment
Cons
Not safe for patients with sulfa allergies
Contraindicated in G6PD deficiency
Risk of severe skin reactions (SJS/TEN)
Not effective for strep throat
Flagyl
Flagyl is a go-to treatment for anaerobic and parasitic infections, and some regimens need only a single dose. Its long history of use has produced a well-understood safety profile, though it carries a boxed warning about cancer risk seen in animal studies. Patients must avoid alcohol during and shortly after treatment.
Pros
Single-dose option for trichomoniasis
Effective against hard-to-treat anaerobic bacteria
Decades of established clinical use
IV and oral forms for flexible dosing
Cons
Boxed warning for carcinogenicity in animal studies
Strict alcohol avoidance required (disulfiram-like reaction)
Risk of seizures and peripheral neuropathy
Brand-name tablets discontinued (generic only)
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