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

When you’re fighting a bacterial infection, your doctor might prescribe cephalexin, a first-generation cephalosporin antibiotic used to treat common bacterial infections like skin boils, urinary tract infections, and respiratory tract infections. Also known as Keflex, it’s one of the most widely used antibiotics because it’s effective, affordable, and generally well-tolerated. Unlike broad-spectrum drugs that hit everything, cephalexin targets specific bacteria—mostly Gram-positive ones like Staphylococcus and Streptococcus—making it a go-to for everyday infections that don’t need heavy artillery.

It’s not magic, though. antibiotic side effects, including diarrhea, nausea, and stomach upset, are common with cephalexin and other antibiotics. Some people even develop yeast infections because the drug wipes out good bacteria along with the bad. And if you’ve ever been told you’re allergic to penicillin, you should talk to your doctor first—there’s a small but real chance of cross-reactivity. It’s not a guarantee you’ll react, but it’s not something to ignore either.

generic antibiotic, cephalexin is one of the most cost-effective options available, often costing less than $10 for a full course. That’s why pharmacies and insurers push it as a first-line choice. But here’s the catch: taking it when you don’t need it—like for a cold or flu—doesn’t help you and hurts everyone else by fueling antibiotic resistance. That’s why doctors are more careful now than they were 20 years ago. They don’t just hand it out. They ask: Is this really a bacterial infection? Could it clear up on its own?

People often wonder how cephalexin stacks up against other antibiotics like amoxicillin or azithromycin. It’s not better or worse—it’s just different. Amoxicillin covers a wider range of bacteria, so it’s often used for ear infections or sinusitis. Azithromycin works better for certain respiratory bugs and doesn’t need to be taken as often. Cephalexin shines when you’ve got a skin infection from a cut or scrape, or a simple UTI. It’s not the strongest, but it’s reliable for the right job.

You’ll also see it mentioned in discussions about bacterial infection, a broad category that includes everything from strep throat to infected insect bites. Not all infections are the same, and not all need antibiotics. But when they do, cephalexin is often the first choice because it’s been around long enough for doctors to know exactly how it behaves in the body. It’s absorbed well by the gut, reaches tissues effectively, and doesn’t need special storage like some other drugs.

What you won’t find in most ads or drug labels is how often people stop taking it too soon. If you feel better after two days, you might be tempted to toss the rest. Don’t. Stopping early lets the toughest bacteria survive and multiply. That’s how superbugs start. Cephalexin usually takes 7 to 10 days to fully clear an infection—even if you’re feeling fine by day three. Finish the whole bottle.

Below, you’ll find real-world guides on how cephalexin fits into the bigger picture of antibiotics—what works, what doesn’t, and what you need to ask your doctor before you even fill the prescription. From how it interacts with other meds to why some people get rashes and others don’t, these posts cut through the noise and give you straight answers.