Cephalexin: What It Treats, How to Take It, and What to Watch For

If you just got handed a prescription for cephalexin and you’re looking it up before your first dose, good instinct. Here’s the plain version of what it is and what to expect.

Cephalexin is an antibiotic. It’s been around a long time, it’s one of the more commonly prescribed drugs in the US, and it used to go by the brand name Keflex, though these days it’s almost always dispensed as a generic. It belongs to a family called cephalosporins, which are chemically related to penicillin. That relationship matters later, so hold onto it.

What It Actually Does

Cephalexin works by stopping bacteria from building their cell walls. No wall, no survival, no multiplying. That’s the whole mechanism, and it’s why it only works on bacteria.

Which brings up the thing worth saying clearly: it does nothing for colds, the flu, or any other viral infection. If you’re taking it for a cough that turned out to be viral, it won’t help, and you’ll get the side effects without the benefit.

What It’s Prescribed For

Doctors reach for cephalexin fairly often because it covers a lot of common ground:

• Skin infections, including cellulitis
• Urinary tract infections
• Strep throat
• Ear infections
• Upper respiratory infections
• Bone infections

It’s approved for adults and for children from about age one and up. That said, it isn’t right for every infection in those categories, since some bacteria have developed resistance to it. Your doctor is making a judgment call based on what’s likely causing your specific infection.

How It’s Taken

Cephalexin comes as capsules, tablets, or a liquid suspension. Capsules and tablets typically come in 250 mg and 500 mg strengths, with 750 mg capsules also available.

Most people take it two to four times a day, often every six or twelve hours depending on the condition, usually for somewhere between 7 and 14 days. You can take it with or without food. If it’s upsetting your stomach, food helps.

A few practical things:

Space the doses evenly. Taking it around the same times each day keeps a steady level in your system, which is the whole point.

Finish the whole course. Even when you feel better on day three. Stopping early is one of the main ways partially treated infections come back tougher than before, and it’s a real contributor to antibiotic resistance.

Store the liquid in the fridge. Capsules and tablets are fine at room temperature, but the suspension needs refrigeration, and you throw out whatever’s left after 14 days.

If you’re still having symptoms after finishing the full course, that’s a call to your doctor, not a reason to keep taking leftover pills.

Side Effects

Most people get through a course of cephalexin without much trouble. When side effects do show up, they’re usually digestive.

Common: Nausea, vomiting, diarrhea, stomach pain or cramps, heartburn. Headache also comes up. Taking it with food often helps with the stomach ones.

Worth calling about promptly: A severe skin reaction, meaning skin pain, fever, sore throat, or a red or purple rash that starts blistering or peeling. Also any signs of an allergic reaction: hives, itching, swelling of the face, lips, tongue, or throat, or trouble breathing or swallowing. Rarely, cephalexin can trigger anaphylaxis, which is a medical emergency.

Watch for afterward, too: Severe or watery diarrhea can be a sign of a C. difficile infection, and this one can show up weeks after you’ve already finished the antibiotic. If you get significant diarrhea during treatment or in the couple of months following it, mention the antibiotic course to your doctor.

The Penicillin Allergy Question

This comes up constantly, so let’s be specific about it.

Cephalexin is chemically similar to penicillin, which means people with a penicillin allergy have a somewhat higher chance of reacting to it too. The overlap is smaller than doctors used to believe, and plenty of people with a penicillin allergy label take cephalosporins without any issue. But it’s still a real consideration.

The practical upshot: tell your prescriber about any penicillin or antibiotic allergy before you start. This isn’t a formality. They may still prescribe it, but they want to know. And if you’ve previously reacted to cephalexin itself or to related drugs like cefuroxime, cefprozil, or cefdinir, that’s a harder stop.

Other Things to Mention to Your Doctor

Kidney problems. Cephalexin is cleared through the kidneys, so reduced kidney function can mean a dosage adjustment.

Blood thinners. There can be an interaction worth monitoring.

Metformin. Another one your doctor will want to know about.

Probenecid. This affects how your body clears cephalexin.

Pregnancy and breastfeeding. There haven’t been adequate controlled studies in pregnancy, so it’s generally used only when clearly needed. It does pass into breast milk, so it’s used with caution while nursing. Either situation is worth a direct conversation rather than a guess.

Birth control. Antibiotics can alter gut bacteria in ways that theoretically affect oral hormonal contraception. The risk is considered low, but if it’s a concern for you, ask your pharmacist about backup during the course.

A Few Common Questions

How fast does it work? Most people start feeling better within a couple of days. If you’re three days in with no improvement at all, or you’re getting worse, call your doctor rather than waiting it out.

Can I drink alcohol on it? Cephalexin doesn’t have the sharp alcohol interaction that some antibiotics do. That said, alcohol can worsen the stomach side effects and won’t help you recover from an infection, so it’s not the week for it.

Is the vet version the same drug? Cephalexin is prescribed for animals too, and it’s the same compound, but the formulations and doses are different. Human and veterinary prescriptions aren’t interchangeable in either direction.

What if I miss a dose? Take it when you remember, unless it’s nearly time for the next one. Don’t double up to catch up.