Keflex vs. Flagyl

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

Key Takeaways

  • Keflex targets aerobic bacteria like strep and staph, while Flagyl targets anaerobic bacteria and parasites such as trichomoniasis, so they treat different infections.

  • Keflex is usually dosed two to four times daily for 7 to 14 days, while Flagyl's course can be a single 2 gram dose or run up to 10 days depending on the infection.

  • Flagyl carries a boxed warning for carcinogenicity in animal studies and reacts dangerously with alcohol, while Keflex's main added risk is C. diff-associated diarrhea.

Keflex vs. Flagyl Drug Summary

Keflex

Keflex (cephalexin) is a first-generation cephalosporin antibiotic used to treat bacterial infections of the skin, respiratory tract, bones, and urinary tract. It comes as an oral capsule, tablet, or liquid suspension taken multiple times a day. It has been used since 1971 and remains one of the most commonly prescribed oral antibiotics.

Flagyl

Flagyl (metronidazole) is a nitroimidazole antimicrobial used to treat trichomoniasis, amebiasis, and infections caused by anaerobic bacteria. It comes as an oral tablet taken multiple times a day, plus capsule, extended-release, and IV forms. Approved since 1963, it is also used off-label for bacterial vaginosis.

Keflex vs. Flagyl Side Effects

Keflex

Most common
Vomiting (rate not quantified in current label)
Nausea (rate not quantified in current label)
Diarrhea (most frequent, rate not quantified)
Dyspepsia (rate not quantified in current label)

Flagyl

Most common
Vomiting (reported, exact % not specified)
Nausea (about 12%)
Diarrhea (reported, exact % not specified)
Headache (reported, exact % not specified)

Keflex vs. Flagyl Dosage

Keflex

Dosage
Dosage Table
STAGE DOSE WHEN
Starting dose 25-50 mg/kg/day divided At the start
Maximum dose up to 4 g/day divided 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

Keflex vs. Flagyl Indications

Keflex

Keflex is FDA-approved for common bacterial infections of the respiratory tract, ears, skin, bones, and urinary tract. It is sometimes used off-label to prevent infection after certain surgeries. It does not treat infections caused by Pseudomonas or Acinetobacter species.

  • Respiratory tract infections

  • Otitis media (ear infections)

  • Skin and skin structure infections

  • Bone infections

  • Genitourinary tract infections

Flagyl

Flagyl is FDA-approved for trichomoniasis, amebiasis, and infections caused by anaerobic bacteria. Sexual partners of people with trichomoniasis are also treated, even without symptoms. It is commonly used off-label for bacterial vaginosis, though that specific dosing regimen is not FDA-approved.

  • Symptomatic and asymptomatic trichomoniasis

  • Treatment of trichomoniasis in sexual partners

  • Amebiasis (intestinal parasite infection)

  • Anaerobic bacterial infections

  • Bacterial vaginosis (off-label)

Keflex vs. Flagyl Pros and Cons

Keflex

Keflex covers common gram-positive skin and respiratory pathogens with a long safety record and no boxed warnings. It is generally well tolerated, though C. diff-associated diarrhea remains a risk with any antibiotic course. Unlike Flagyl, it does not interact dangerously with alcohol.

Pros

  • No boxed warning or alcohol restriction

  • Long track record of safety since 1971

  • Available as capsule, tablet, or liquid suspension

Cons

  • Not effective against anaerobic bacteria or parasites

  • Requires dosing 2 to 4 times daily

  • Can trigger false-positive urine glucose tests

  • Increases metformin levels, raising hypoglycemia risk

Flagyl

Flagyl is highly effective against anaerobic bacteria and certain parasitic infections that Keflex cannot treat, and some regimens need only a single dose. It carries a boxed warning for carcinogenicity seen in animal studies and cannot be combined with alcohol. It is also contraindicated in people with Cockayne syndrome or recent disulfiram use.

Pros

  • Some infections need only a single 2 g dose

  • Effective against anaerobic bacteria and parasites

  • Multiple formulations including extended-release and IV

Cons

  • Boxed warning for carcinogenicity in animal studies

  • Cannot be combined with alcohol or disulfiram

  • Contraindicated in Cockayne syndrome due to liver risk

  • Risk of seizures, encephalopathy, and neuropathy

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