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

Most common
anorexia (rate not quantified)
nausea (rate not quantified)
vomiting (rate not quantified)
rash (rate not quantified)

Flagyl

Most common
anorexia (rate not specified)
nausea (about 12%)
headache (rate not specified)
diarrhea (rate not specified)

Bactrim vs. Flagyl Dosage

Bactrim

Dosage
Dosage Table
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

Dosage
Dosage Table
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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