Metronidazole vs. Keflex

See how Metronidazole and Keflex compare on approved uses, dosing, side effects, and cost.

Key Takeaways

  • Metronidazole targets parasites, anaerobic bacteria, and vaginal infections, while Keflex treats skin, respiratory, bone, and urinary infections from susceptible bacteria.

  • Metronidazole carries a boxed warning about carcinogenicity in animal studies and requires strict alcohol avoidance, while Keflex's main risks are allergic reactions and C. diff-associated diarrhea.

  • Both are affordable generics, but metronidazole is typically the less expensive option per course of treatment.

Metronidazole vs. Keflex Drug Summary

Metronidazole

Metronidazole is a nitroimidazole antimicrobial used for trichomoniasis, bacterial vaginosis, amebiasis, and infections caused by anaerobic bacteria. It's usually taken as an oral tablet or capsule several times a day for about a week, and it also comes in IV, topical gel, and vaginal gel forms. It has been used since 1963 and remains a first-line treatment for several parasitic and anaerobic infections.

Keflex

Keflex (cephalexin) is a first-generation cephalosporin antibiotic used for respiratory, skin, bone, and urinary tract infections caused by susceptible bacteria. It's taken orally as a capsule, tablet, or liquid, usually every 6 to 12 hours for 7 to 14 days. Approved in 1971, it remains one of the most widely prescribed oral cephalosporins.

Metronidazole vs. Keflex Side Effects

Metronidazole

Most common
vomiting (occasionally reported)
nausea (most common)
diarrhea (commonly reported)
headache (commonly reported)

Keflex

Most common
vomiting (rate not specified)
nausea (rate not specified)
diarrhea (most frequent)
dyspepsia (rate not specified)

Metronidazole vs. Keflex Dosage

Metronidazole

Dosage
Dosage Table
STAGE DOSE WHEN
Starting dose 250 mg three times daily Days 1-7
Maximum dose 2 g single or divided dose Day 1
Dose changes 1 increase As tolerated

Keflex

Dosage
Dosage Table
STAGE DOSE WHEN
Starting dose 25-50 mg/kg/day divided At the start
Maximum dose 25-50 mg/kg/day divided Same as starting
Dose changes None Fixed dose

Metronidazole vs. Keflex Indications

Metronidazole

Metronidazole is FDA-approved for trichomoniasis, bacterial vaginosis, amebiasis, and serious anaerobic bacterial infections such as intra-abdominal infections. It's also used off-label for H. pylori eradication and C. difficile infections. It is not effective against viral infections like the common cold.

  • Trichomoniasis (symptomatic and asymptomatic)

  • Bacterial vaginosis

  • Amebiasis

  • Intra-abdominal infections (peritonitis, abscess)

  • Anaerobic bacterial infections

Keflex

Keflex is FDA-approved for respiratory tract infections, otitis media, skin and skin structure infections, bone infections, and genitourinary tract infections caused by susceptible bacteria. It's sometimes used off-label to help prevent surgical site infections. It is not effective against Pseudomonas or Acinetobacter species.

  • Respiratory tract infections

  • Otitis media

  • Skin and skin structure infections

  • Bone infections

  • Genitourinary tract infections

Metronidazole vs. Keflex Pros and Cons

Metronidazole

Metronidazole works well against parasites and anaerobic bacteria and comes in several forms for different infection sites. Its long history of use has produced a well-documented safety profile, though it carries a boxed warning about carcinogenicity seen in animal studies and requires strict alcohol avoidance.

Pros

  • First-line option for trichomoniasis and bacterial vaginosis

  • Available in oral, IV, topical, and vaginal forms

  • Long track record of use dating back to 1963

  • Roughly $15 to $40 per month, less expensive than Keflex

Cons

  • Boxed warning for carcinogenicity shown in animal studies

  • Cannot be combined with alcohol due to severe reaction risk

  • Contraindicated with disulfiram and in Cockayne syndrome

  • Risk of neurological effects like seizures and neuropathy

Keflex

Keflex covers many common bacterial infections affecting the skin, bones, and respiratory tract with a well-established safety record. It does not carry metronidazole's alcohol restriction or carcinogenicity warning, but it is not active against certain resistant bacteria like Pseudomonas.

Pros

  • Effective against a broad range of skin, bone, and respiratory infections

  • No restriction on alcohol use during treatment

  • Widely prescribed with a long safety record since 1971

  • Available as capsule, tablet, or liquid for flexible dosing

Cons

  • Not effective against Pseudomonas or Acinetobacter infections

  • Contraindicated in patients allergic to cephalosporins

  • Can raise metformin levels, increasing hypoglycemia risk

  • Roughly $53 to $95 per short course, pricier than metronidazole

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