Keflex (cephalexin) is not universally stronger than amoxicillin. Each antibiotic outperforms the other for different types of infections, which is why doctors choose between them based on what you’re being treated for, not on overall “strength.” For skin infections, Keflex has a clear edge. For respiratory infections, amoxicillin is the better drug.
How These Antibiotics Work
Keflex and amoxicillin belong to different antibiotic families but kill bacteria in nearly identical ways. Both destroy bacteria by blocking an enzyme that builds the bacterial cell wall, causing the cell to burst. Keflex is a first-generation cephalosporin, while amoxicillin is an aminopenicillin, a subclass of penicillin. This family difference matters because it determines which bacteria each drug can reach effectively.
Where Keflex Has the Advantage
Keflex is the stronger choice for skin and soft tissue infections. It covers both Strep (the most common cause of cellulitis) and staph bacteria that are methicillin-susceptible, meaning the majority of everyday staph infections you’d pick up from a cut or scrape. Amoxicillin, by contrast, is not effective against Staphylococcus aureus in most cases. That gap makes Keflex more versatile for skin problems.
Infectious disease guidelines from the IDSA specifically recommend cephalexin (alongside dicloxacillin) as a first-line treatment for impetigo and other superficial skin infections caused by staph. For cellulitis, both drugs appear on the recommended list, but Keflex covers a broader range of the bacteria likely to be involved. If your doctor suspects a staph component to a skin infection, Keflex is the logical pick over amoxicillin alone.
Where Amoxicillin Has the Advantage
For respiratory infections, amoxicillin is the stronger antibiotic. It is more potent against Streptococcus pneumoniae, the bacterium behind most cases of bacterial sinusitis, middle ear infections, and community-acquired pneumonia. A review of oral antibiotics found that cephalexin was among the least potent options with the narrowest spectrum against common respiratory pathogens, while amoxicillin (especially when paired with clavulanate) was the most active against S. pneumoniae, including strains that have developed some resistance to penicillin.
Amoxicillin also has better bioavailability than many comparable antibiotics when taken by mouth, meaning your body absorbs a higher percentage of each dose. The Johns Hopkins ABX Guide notes that amoxicillin is often preferred over penicillin V for oral treatment partly for this reason. For strep throat, ear infections, and sinus infections, amoxicillin remains the standard first-line choice in most clinical guidelines.
Bacterial Coverage at a Glance
- Keflex: Covers Group A Strep and methicillin-susceptible Staph aureus (MSSA). Does not cover MRSA. Weaker against respiratory pathogens like S. pneumoniae.
- Amoxicillin: Excellent against Group A Strep and S. pneumoniae. Poor against most Staph aureus. Does not cover MRSA.
Neither antibiotic works against MRSA. If your doctor suspects a MRSA infection, a different class of antibiotic is needed entirely.
Side Effects Compared
Both antibiotics cause similar digestive side effects, and neither is dramatically worse than the other. Diarrhea and nausea are the most common complaints with both drugs. Based on user-reported data from Drugs.com, Keflex tends to cause slightly more nausea (about 15% vs. 13% for amoxicillin), headaches (about 12% vs. 7%), and dizziness (about 9%, which is less commonly reported with amoxicillin). Amoxicillin users report yeast infections somewhat more often.
Rare but serious reactions can occur with either drug, including allergic reactions like facial swelling, breathing difficulty, and skin rashes. Keflex carries additional cautions for people with seizure disorders or liver disease, while amoxicillin has a well-known interaction with mononucleosis, frequently causing a widespread rash in people with active mono.
Penicillin Allergy and Keflex
If you’re allergic to amoxicillin or penicillin, Keflex is sometimes offered as an alternative, but it’s not risk-free. First-generation cephalosporins like Keflex have a cross-reactivity rate of 1% to 8% in people with confirmed penicillin allergies, according to CDC data. That means a small but real percentage of penicillin-allergic patients will also react to Keflex. Third-generation cephalosporins have a lower cross-reactivity rate (under 1%), but Keflex is not in that category. If you’ve had a serious allergic reaction to penicillin, your doctor will likely want to verify your allergy status before prescribing any cephalosporin.
Dosing Differences
One practical difference: Keflex typically needs to be taken more frequently. The standard adult dose is 250 to 500 mg four times daily, while amoxicillin is usually taken 250 to 500 mg three times daily. For some infections, amoxicillin can even be dosed twice daily with higher individual doses. If you struggle with remembering multiple doses throughout the day, amoxicillin’s schedule may be easier to follow, and missed doses can reduce how well any antibiotic works.
The total daily dose range also differs. Cephalexin can go up to 4,000 mg per day for severe infections, reflecting the higher per-dose and more frequent dosing schedule. Both antibiotics are inexpensive and widely available as generics.
Which One Your Doctor Will Choose
The choice between Keflex and amoxicillin almost always comes down to the type of infection. For a skin wound, an abscess that’s been drained, or cellulitis with possible staph involvement, Keflex is the more appropriate antibiotic. For strep throat, a sinus infection, an ear infection, or pneumonia, amoxicillin is more effective against the bacteria most likely to be causing the problem.
In some situations, like mild cellulitis that’s clearly caused by Strep alone, either drug could work. But when staph coverage matters, Keflex fills a gap that amoxicillin cannot. And when respiratory pathogens are the target, amoxicillin is more potent at lower doses. Thinking of one as categorically “stronger” misses the point. They’re different tools built for different jobs.