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
Keflex
Bactrim vs. Keflex Dosage
Bactrim
Keflex
| 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 |
| 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
| 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
| 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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