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

When your doctor prescribes Cephalexin, a first-generation cephalosporin antibiotic used to treat bacterial infections like skin boils, urinary tract infections, and respiratory tract infections. Also known as Keflex, it works by stopping bacteria from building cell walls, which kills them or stops them from spreading. Unlike some antibiotics, Cephalexin is often chosen for people with a penicillin allergy—though not always safe, it’s one of the more commonly used alternatives when penicillin isn’t an option.

It’s not just about taking a pill. Cephalexin’s effectiveness depends on the type of bacteria you’re fighting. It’s great against common ones like Staphylococcus and Streptococcus, but useless against viruses like the flu or colds. If you’ve ever taken it for a skin infection and felt better in a couple of days, that’s why—it targets the bugs causing the redness, swelling, or pus. But if you stop too early, even if you feel fine, the surviving bacteria can come back stronger. That’s why finishing the full course matters more than most people realize.

Side effects are usually mild—upset stomach, diarrhea, or a bad taste in your mouth—but for some, it triggers allergic reactions like rashes or swelling. If you’ve had a serious reaction to penicillin, your doctor will check carefully before prescribing Cephalexin. It’s not a 100% safe swap. And while it’s often used for kids and older adults, kidney function matters. If your kidneys aren’t working well, the drug can build up and cause problems. That’s why your doctor might lower the dose or space out the pills.

What’s interesting is how often Cephalexin shows up in discussions about antibiotic misuse. People sometimes save leftover pills for the next time they feel sick. That’s dangerous. Not only does it risk incomplete treatment, but it also contributes to antibiotic resistance—a growing global problem. The posts below show real cases: someone using it for a tooth infection without a diagnosis, another person combining it with alcohol and wondering why they felt worse, and even a parent confused about why their child’s ear infection didn’t improve after five days. These aren’t hypotheticals—they’re the kinds of stories you’ll find here.

You’ll also see comparisons with other antibiotics like amoxicillin, clindamycin, and azithromycin. Why pick Cephalexin over them? Cost, dosing schedule, allergy history, and the infection type all play a role. One post breaks down how Cephalexin is often cheaper than newer options and taken four times a day, while others like azithromycin are taken once daily but cost more. Another dives into what happens when Cephalexin doesn’t work—what’s next? And how do you know if it’s the drug or the bug that’s the problem?

There’s no magic bullet in antibiotics. Cephalexin is useful, but only when it’s the right tool for the job. The posts below give you the real talk: what works, what doesn’t, and what to watch out for. No fluff. No marketing. Just what patients and providers actually experience.