Cephalexin vs Amoxicillin: Key Differences and When to Use Each

When your doctor prescribes an antibiotic for a bacterial infection, you might hear cephalexin, a first-generation cephalosporin antibiotic used for skin, bone, and respiratory infections. Also known as Keflex, it works by stopping bacteria from building cell walls. Or you might get amoxicillin, a penicillin-class antibiotic commonly used for ear infections, sinusitis, and pneumonia. It’s one of the most prescribed antibiotics in the world, and for good reason. Both fight bacteria, but they’re not interchangeable—and choosing the wrong one can mean a longer illness or worse side effects.

Amoxicillin is often the first choice for common infections like strep throat or ear infections because it’s broad-spectrum and well-tolerated. But if you’re allergic to penicillin, amoxicillin is off the table. That’s where cephalexin, a non-penicillin antibiotic that’s safe for many with penicillin allergies comes in. It’s not a perfect substitute—some bacteria resistant to penicillin are also resistant to cephalexin—but for many people, it’s the next best option. You’ll also see cephalexin used more often for skin infections like cellulitis or boils, while amoxicillin is preferred for respiratory bugs like bronchitis or sinus infections. Dosing differs too: amoxicillin often needs to be taken three times a day, while cephalexin is usually twice daily, which can make adherence easier.

Side effects overlap—nausea, diarrhea, upset stomach—but amoxicillin can cause more rash in people with mononucleosis, and cephalexin might lead to more yeast infections in women. Neither works for viruses, so if you’ve got a cold or the flu, neither will help. And both can contribute to antibiotic resistance if used too often or for too long. That’s why doctors don’t just pick one at random—they look at your infection type, your allergy history, your age, and even your kidney function. If you’ve been on one and it didn’t work, the other might be next. But don’t switch on your own. The right choice isn’t about which is stronger—it’s about which one matches your specific bug and your body’s needs.

Below, you’ll find real comparisons, patient experiences, and prescribing insights from posts that break down exactly when each drug shines—and when it falls short. Whether you’re dealing with a stubborn skin infection, a recurring sinus issue, or just trying to understand why your doctor picked one over the other, the articles here give you the facts without the fluff.