Bactrim vs. Keflex

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

Key Takeaways

  • Bactrim is a sulfonamide combination antibiotic taken twice daily for most infections, while Keflex is a cephalosporin usually taken two to four times daily.

  • Both treat certain urinary tract infections, but they belong to different drug classes with different allergy risks and safety profiles.

  • Bactrim carries specific warnings around sulfa allergy and severe skin reactions, while Keflex carries cross-reactivity risk with penicillin allergy and C. diff risk.

Bactrim vs. Keflex Drug Summary

Bactrim

Bactrim combines sulfamethoxazole and trimethoprim, two antibacterials that block sequential steps in bacterial folate production. It is FDA-approved for urinary tract infections, traveler's diarrhea, and Pneumocystis pneumonia, among other uses. It is usually taken as an oral tablet or suspension every 12 hours.

Keflex

Keflex (cephalexin) is a first-generation cephalosporin antibiotic that stops bacteria from building their cell walls. It is FDA-approved for respiratory, skin, bone, and urinary tract infections. It is typically taken as an oral capsule every 6 to 12 hours depending on the infection.

Bactrim vs. Keflex Side Effects

Bactrim

Most common
Vomiting (rate not specified)
Nausea (rate not specified)
Diarrhea (rate not specified)
Anorexia (rate not specified)

Keflex

Most common
Vomiting (rate not specified)
Nausea (rate not specified)
Diarrhea (rate not specified)
Dyspepsia (rate not specified)

Bactrim vs. Keflex Dosage

Bactrim

Dosage
Dosage Table
STAGE DOSE WHEN
Starting dose 800 mg/160 mg every 12h At the start
Maximum dose 8-10 mg/kg/day div q6-12h If needed
Dose changes 3 increases As tolerated

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

Bactrim vs. Keflex Indications

Bactrim

Bactrim is FDA-approved for treating urinary tract infections caused by susceptible bacteria, along with other infections like shigellosis and Pneumocystis pneumonia. It is also used off-label for certain MRSA skin infections. It is not appropriate for strep throat.

  • Urinary tract infections

  • Acute otitis media in children

  • Traveler's diarrhea in adults

  • Pneumocystis jirovecii pneumonia

  • Shigellosis

Keflex

Keflex is FDA-approved for infections of the respiratory tract, skin, bone, and urinary tract caused by susceptible bacteria. It is a common choice for uncomplicated UTIs and skin infections. It is not effective against Pseudomonas or Acinetobacter species.

  • Genitourinary tract infections

  • Skin and skin structure infections

  • Respiratory tract infections

  • Bone infections

  • Otitis media

Bactrim vs. Keflex Pros and Cons

Bactrim

Bactrim is one of the least expensive antibiotics available and covers a broad range of bacteria including some MRSA strains off-label. It requires twice-daily dosing for most infections, which some patients find easier to remember. Its sulfa component carries specific allergy and blood-related risks that Keflex does not share.

Pros

  • Typically dosed just twice a day

  • Roughly $10 to $30 per month, notably cheaper than Keflex

  • Covers some MRSA strains off-label

  • Effective for traveler's diarrhea and PCP pneumonia

Cons

  • Contraindicated in G6PD deficiency due to hemolytic anemia risk

  • Higher risk of severe skin reactions like Stevens-Johnson syndrome

  • Not safe during pregnancy or breastfeeding

  • Not an option for patients with sulfa allergy

Keflex

Keflex has a long safety record and is generally well tolerated, with a lower risk of the severe skin reactions seen with sulfa drugs. It usually requires dosing two to four times a day depending on the infection. It costs more out of pocket than Bactrim, roughly $53 to $95 for a typical course.

Pros

  • Lower risk of severe cutaneous drug reactions

  • Considered safer during pregnancy

  • Fewer major drug interactions to monitor

  • Long track record of use in children

Cons

  • Usually needs dosing two to four times a day

  • Roughly $53 to $95 per course, pricier than Bactrim

  • Carries C. diff diarrhea risk with prolonged use

  • Cross-reactivity possible in penicillin-allergic patients

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