Amoxicillin vs. Metronidazole

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

Key Takeaways

  • Amoxicillin is a penicillin-class antibiotic used for ear, throat, respiratory, skin, and urinary infections, while metronidazole targets anaerobic bacteria and parasites like trichomoniasis and bacterial vaginosis.

  • Both are inexpensive generic antibiotics, but metronidazole carries a boxed warning for carcinogenicity in animal studies and cannot be combined with alcohol.

  • Amoxicillin is usually dosed two to three times daily for days to weeks, while metronidazole's schedule ranges from a single dose to a week-long three-times-daily course depending on the infection.

Amoxicillin vs. Metronidazole Drug Summary

Amoxicillin

Amoxicillin is an aminopenicillin antibiotic that treats bacterial infections of the ear, throat, sinuses, lungs, skin, and urinary tract. It also treats gonorrhea and, in combination regimens, H. pylori infection. It's taken orally as a capsule, tablet, chewable tablet, or liquid suspension, usually two to three times a day.

Metronidazole

Metronidazole is a nitroimidazole antimicrobial that treats parasitic and anaerobic bacterial infections, including trichomoniasis, bacterial vaginosis, amebiasis, and intra-abdominal infections. It's also used off-label for H. pylori and C. difficile. It comes as an oral tablet or capsule, with dosing that ranges from a single dose to a week of three-times-daily treatment.

Amoxicillin vs. Metronidazole Side Effects

Amoxicillin

Most common
nausea (greater than 1%)
diarrhea (greater than 1%)
vomiting (greater than 1%)
rash (greater than 1%)

Metronidazole

Most common
nausea (rate not specified)
diarrhea (commonly reported)
metallic taste (commonly reported)
headache (commonly reported)

Amoxicillin vs. Metronidazole Dosage

Amoxicillin

Dosage
Dosage Table
STAGE DOSE WHEN
Starting dose 250 mg every 8 hours At the start
Maximum dose 1 g three times daily If needed
Dose changes 4 increases As tolerated

Metronidazole

Dosage
Dosage Table
STAGE DOSE WHEN
Starting dose 250 mg three times daily Days 1-7
Maximum dose 500-750 mg, 3x daily If needed
Dose changes 2 increases As tolerated

Amoxicillin vs. Metronidazole Indications

Amoxicillin

Amoxicillin is FDA-approved for common bacterial infections affecting the ears, nose, throat, lungs, skin, and urinary tract. It's also approved for uncomplicated gonorrhea and, combined with other drugs, for eradicating H. pylori. It is not effective against viral infections.

  • Ear, nose, and throat infections

  • Lower respiratory tract infections

  • Skin and skin structure infections

  • Urinary tract infections

  • H. pylori eradication (combination therapy)

Metronidazole

Metronidazole is FDA-approved for trichomoniasis, bacterial vaginosis, amebiasis, and anaerobic bacterial infections, including intra-abdominal infections. It's also commonly used off-label for H. pylori and C. difficile infections. It is not effective against viral infections.

  • Trichomoniasis

  • Bacterial vaginosis

  • Amebiasis

  • Intra-abdominal infections (peritonitis, abscess)

  • H. pylori and C. difficile (off-label)

Amoxicillin vs. Metronidazole Pros and Cons

Amoxicillin

Amoxicillin has a long safety record and is one of the most affordable, widely prescribed antibiotics available. It works well for common respiratory, ear, and urinary infections but carries a risk of allergic reactions, including rare anaphylaxis.

Pros

  • Available as chewable tablets and liquid for children

  • Low cost, widely available generic

  • Fewer serious boxed warnings than metronidazole

  • Effective for common ear, sinus, and respiratory infections

Cons

  • Risk of allergic reaction, including anaphylaxis

  • Not effective against anaerobic bacteria or parasites

  • Can cause severe rash in patients with mononucleosis

  • Bacterial resistance is increasingly common

Metronidazole

Metronidazole is a first-line treatment for trichomoniasis and bacterial vaginosis and covers infections amoxicillin cannot treat, such as anaerobic bacteria and parasites. It carries a boxed warning for carcinogenicity in animal studies and cannot be combined with alcohol.

Pros

  • First-line therapy for trichomoniasis and bacterial vaginosis

  • Covers anaerobic bacteria and certain parasites

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

  • Useful off-label for C. difficile and H. pylori

Cons

  • Boxed warning for carcinogenicity in animal studies

  • Causes disulfiram-like reaction with alcohol

  • Risk of seizures, neuropathy, and metallic taste

  • Contraindicated with disulfiram and Cockayne syndrome

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