Cephalexin vs. Flagyl

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

Key Takeaways

  • Cephalexin is a cephalosporin antibiotic used for skin, respiratory, bone, and urinary infections caused by susceptible bacteria.

  • Flagyl (metronidazole) targets anaerobic bacteria and parasites, treating infections like trichomoniasis, amebiasis, and certain anaerobic infections.

  • The two drugs are rarely interchangeable because they target different types of organisms, though both are older, well-established generics.

Cephalexin vs. Flagyl Drug Summary

Cephalexin

Cephalexin is a first-generation cephalosporin antibacterial used for infections like strep throat, skin infections, and urinary tract infections. It comes as a capsule, tablet, or oral suspension, typically taken two to four times a day. It has been used safely since the early 1970s and is only available as a generic.

Flagyl

Flagyl (metronidazole) is a nitroimidazole antimicrobial used mainly for trichomoniasis, amebiasis, and anaerobic bacterial infections. It comes as an oral tablet, capsule, extended-release tablet, or IV form, usually taken multiple times a day for one to two weeks. Alcohol must be avoided during treatment and for several days afterward.

Cephalexin vs. Flagyl Side Effects

Cephalexin

Most common
vomiting (rate not specified in FDA label)
nausea (rate not specified in FDA label)
diarrhea (rate not specified in FDA label)
dyspepsia (rate not specified in FDA label)

Flagyl

Most common
vomiting (reported, exact % not specified)
nausea (about 12%)
diarrhea (reported, exact % not specified)
headache (reported, exact % not specified)

Cephalexin vs. Flagyl Dosage

Cephalexin

Dosage
Dosage Table
STAGE DOSE WHEN
Starting dose 25-50 mg/kg/day divided At the start
Maximum dose 1 to 4 g/day divided If needed
Dose changes 4 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

Cephalexin vs. Flagyl Indications

Cephalexin

Cephalexin is FDA-approved for a range of bacterial infections caused by susceptible organisms, including respiratory, skin, bone, and urinary tract infections. It is often chosen for outpatient treatment of common bacterial infections in both adults and children.

  • Respiratory tract infections (strep throat, pneumonia)

  • Otitis media (middle ear infections)

  • Skin and skin structure infections

  • Bone infections

Flagyl

Flagyl is FDA-approved for trichomoniasis, amebiasis, and infections caused by anaerobic bacteria, organisms that thrive without oxygen. It is also commonly prescribed off-label for bacterial vaginosis, though that use follows a different dosing schedule than the FDA-approved indications.

  • Symptomatic and asymptomatic trichomoniasis

  • Amebiasis (intestinal parasite infection)

  • Anaerobic bacterial infections

  • Bacterial vaginosis (off-label use)

Cephalexin vs. Flagyl Pros and Cons

Cephalexin

Cephalexin has a long safety record and broad use across respiratory, skin, bone, and urinary infections. It does not carry the same alcohol restrictions or carcinogenicity warning that metronidazole does, but it is not effective against anaerobic bacteria or parasites.

Pros

  • No alcohol interaction or disulfiram-like reaction risk

  • Available as an oral suspension for children

  • No boxed warning for carcinogenicity

  • Broad coverage for common skin, respiratory, and urinary infections

Cons

  • Not effective against anaerobic bacteria or parasites

  • Dose adjustment needed for kidney impairment (CrCl below 30 mL/min)

  • Carries a risk of C. difficile-associated diarrhea

  • Not effective against trichomoniasis or amebiasis

Flagyl

Flagyl covers infections that cephalexin cannot treat, including anaerobic bacteria and certain parasitic infections. It comes with a boxed carcinogenicity warning and a strict alcohol restriction, along with a risk of neurological side effects with prolonged use.

Pros

  • Effective against anaerobic bacteria and protozoal infections

  • Single 2 g dose option available for trichomoniasis

  • Available in oral, extended-release, and IV forms

  • Standard treatment for amebiasis and BV

Cons

  • Boxed warning for carcinogenicity in animal studies

  • Must avoid alcohol during and for days after treatment

  • Risk of seizures, peripheral neuropathy, and encephalopathy

  • Metallic taste is a common complaint during treatment

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