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

When you have a stubborn skin infection, a bladder infection, or even a bad sinus infection, 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—not because it's the strongest, but because it works well for everyday infections and has a long safety record.

Cephalexin fights bacteria by breaking down their cell walls, which stops them from multiplying. It’s not a magic bullet—it won’t touch viruses like the flu or a cold—but for bacterial issues like strep throat, cellulitis, or urinary tract infections, it’s often the go-to. If you’ve ever been told you can’t take penicillin, cephalexin might still be an option, but not always. About 10% of people allergic to penicillin also react to cephalexin, so it’s not automatic. Always tell your doctor about any past reactions. It’s also one of the few antibiotics that’s safe during pregnancy when needed, which is why it shows up often in women’s health guides.

It’s not just about taking the pill. How you take it matters. Cephalexin works best on an empty stomach, but if it upsets your stomach, food is fine—just don’t take it with dairy if you’re on a strict regimen, as calcium can interfere. Side effects? Mostly mild: diarrhea, nausea, or a rash. But if you get watery diarrhea that won’t stop, or a fever after starting it, that could be C. diff, and you need to stop and call your doctor. Most people finish their 7- to 10-day course without issues, but skipping doses or stopping early is how resistant bacteria start. That’s why this antibiotic is so common in the posts below—people are asking how to use it right, what to avoid, and when it’s not the right choice.

There’s a reason you’ll see cephalexin tied to posts about UTIs, skin infections, and even dental abscesses. It’s not just a drug—it’s part of a bigger picture. People are trying to understand how it compares to amoxicillin, what to do if it doesn’t work, and how to manage side effects without reaching for more meds. You’ll find real stories here: someone who got a rash and wondered if it was an allergy, another who used it after surgery and needed help with gut recovery, and people who switched from penicillin and want to know if it’s safe. This isn’t just a drug listing. It’s a collection of real questions from real people who’ve been there.