Metronidazole vs. Cephalexin

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

Key Takeaways

  • Metronidazole targets anaerobic bacteria and protozoa, making it a go-to for trichomoniasis, bacterial vaginosis, and abdominal infections, while cephalexin targets staph and strep infections of the skin, respiratory tract, and urinary tract.

  • Both drugs are inexpensive, decades-old generics, but metronidazole requires avoiding alcohol entirely due to a severe reaction, while cephalexin carries a cross-allergy risk for people with penicillin allergies.

  • Metronidazole is usually prescribed for short courses of a week or less, while cephalexin's dosing frequency and duration vary more depending on which infection is being treated.

Metronidazole vs. Cephalexin Drug Summary

Metronidazole

Metronidazole is a nitroimidazole antimicrobial used to treat infections caused by anaerobic bacteria and certain protozoa, including trichomoniasis, bacterial vaginosis, and amebiasis. It is usually taken as an oral tablet or capsule two to four times a day, and is also available as an IV, topical, or vaginal formulation.

Cephalexin

Cephalexin is a first-generation cephalosporin antibiotic used to treat bacterial infections of the skin, respiratory tract, ears, bones, and urinary tract. It is taken orally as a capsule, tablet, or liquid suspension, usually every 6 to 12 hours depending on the infection and dose.

Metronidazole vs. Cephalexin Side Effects

Metronidazole

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

Cephalexin

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

Metronidazole vs. Cephalexin Dosage

Metronidazole

Dosage
Dosage Table
STAGE DOSE WHEN
Starting dose 250 mg three times daily Days 1-7
Maximum dose 500-750 mg three times daily If needed
Dose changes 2 increases As tolerated

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

Metronidazole vs. Cephalexin Indications

Metronidazole

Metronidazole is FDA-approved for infections caused by anaerobic bacteria and certain protozoa, such as trichomoniasis, bacterial vaginosis, and amebiasis. It also treats intra-abdominal infections like peritonitis and abscesses. Off-label, it is a key part of combination therapy for H. pylori and C. difficile infections.

  • Trichomoniasis (symptomatic and asymptomatic)

  • Bacterial vaginosis

  • Amebiasis

  • Intra-abdominal infections and abscesses

  • Off-label: H. pylori and C. difficile treatment

Cephalexin

Cephalexin is FDA-approved for infections of the skin, respiratory tract, ears, bones, and urinary tract caused by susceptible staphylococci and streptococci. It also treats genitourinary infections, including acute prostatitis. Off-label, it is sometimes used as an adjunct for acne or to prevent surgical site infections.

  • Skin and skin structure infections

  • Respiratory tract infections

  • Otitis media

  • Urinary tract infections, including prostatitis

  • Bone infections

Metronidazole vs. Cephalexin Pros and Cons

Metronidazole

Metronidazole is a first-line option for trichomoniasis and bacterial vaginosis, with strong activity against anaerobes and protozoa that many other antibiotics lack. It is inexpensive and comes in oral, IV, topical, and vaginal forms, but it carries a boxed warning about carcinogenicity in animal studies and can cause a severe reaction with alcohol.

Pros

  • First-line treatment for trichomoniasis and bacterial vaginosis

  • Effective against anaerobic bacteria and protozoa other antibiotics miss

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

  • Inexpensive, widely available generic

Cons

  • Must avoid alcohol during and after treatment

  • Boxed warning for carcinogenicity seen in animal studies

  • Can cause a persistent metallic taste

  • Risk of seizures or nerve damage with prolonged use

Cephalexin

Cephalexin is a familiar, well-tolerated choice for common skin, respiratory, ear, and urinary infections caused by susceptible staph and strep bacteria. It has been used safely for decades and does not require avoiding alcohol, but it is not effective against anaerobic bacteria or protozoa, and people with penicillin allergies may react to it too.

Pros

  • Broad activity against common staph and strep infections

  • No alcohol restriction during treatment

  • Long track record of safety since the early 1970s

  • Available as capsule, tablet, or liquid suspension

Cons

  • Not effective against anaerobic bacteria or protozoal infections

  • Cross-reactivity risk for patients with penicillin allergy

  • Dose must be adjusted for reduced kidney function

  • Dosing every 6 to 12 hours can be less convenient than shorter courses

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