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
Flagyl
Keflex vs. Flagyl Dosage
Keflex
Flagyl
| 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 |
| 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
| 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
| 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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