Cephalexin: What It Is, How It Works, and What You Need to Know

When you’ve got a stubborn skin infection, a bad sinus infection, or a urinary tract bug that won’t quit, your doctor might reach for cephalexin, a first-generation cephalosporin antibiotic used to treat common bacterial infections. Also known as Keflex, it’s one of the most prescribed antibiotics in the U.S. and Canada because it works fast, is usually well-tolerated, and costs very little. Unlike some broad-spectrum drugs that hit everything in sight, cephalexin targets specific bacteria — mostly gram-positive ones like Staph and Strep — making it a smart first pick for many everyday infections.

It’s not the only antibiotic out there, but it’s often the go-to when you need something stronger than penicillin but don’t want to go full-spectrum. People often compare it to amoxicillin, a penicillin-class antibiotic commonly used for ear infections, pneumonia, and strep throat. Both are oral, both are affordable, but cephalexin holds up better against certain resistant strains. If you’re allergic to penicillin, cephalexin is often a safe alternative — though not always. About 1 in 10 people with penicillin allergies react to cephalosporins too, so talk to your doctor before starting.

Cephalexin doesn’t work on viruses, so don’t use it for colds or flu. But for bacterial infections like cellulitis, bone infections, or even some cases of acne, it’s a solid tool. Side effects? Most people feel fine, but stomach upset, diarrhea, and nausea happen. Rarely, it can cause a serious gut infection called C. diff. If you get watery diarrhea that doesn’t go away, stop taking it and call your doctor. Also, don’t skip doses just because you feel better. Stopping early is how superbugs grow.

You’ll often see cephalexin listed alongside other antibiotics in medical guides — like chloramphenicol, an older broad-spectrum antibiotic used in low-resource settings despite serious risks, or azithromycin, a macrolide antibiotic often used for respiratory and skin infections. But cephalexin stands out because it’s reliable, predictable, and has been around since the 1960s without losing its edge. It’s not flashy, but it gets the job done.

What you’ll find in the posts below are real comparisons, patient experiences, and practical advice about using cephalexin safely. You’ll see how it stacks up against other antibiotics, what to do if it doesn’t work, how to handle side effects, and even how to avoid common mistakes like taking it with dairy or skipping doses. No fluff. No hype. Just clear, straight talk from people who’ve been there — and from experts who know what actually works.