Cephalexin Side Effects: What You Need to Know Before Taking It

When you take cephalexin, a first-generation cephalosporin antibiotic used to treat bacterial infections like skin infections, urinary tract infections, and respiratory infections. Also known as Keflex, it's one of the most commonly prescribed antibiotics in North America. Most people tolerate it fine, but not everyone does. Side effects aren’t rare—they happen in up to 1 in 10 users. Some are just annoying, like nausea or diarrhea. Others, like severe allergic reactions, can be life-threatening if ignored.

One of the biggest risks with cephalexin, a beta-lactam antibiotic related to penicillin. Also known as Keflex, it is cross-reactivity with penicillin allergies. If you’ve had a rash, swelling, or trouble breathing after taking penicillin, you might react to cephalexin too. Doctors often skip it for those patients. But even if you’ve never had a reaction before, watch for signs: hives, itching, swelling of the face or throat, or sudden wheezing. Don’t wait—call your doctor or go to the ER. Another common issue is antibiotic-associated diarrhea, caused by disruption of gut bacteria, sometimes leading to C. diff infection. If you get watery stools, cramps, or fever after starting cephalexin, don’t just take anti-diarrhea meds. That could hide a dangerous infection.

Some side effects are less obvious. Long-term use can lead to yeast infections in women—or even in men. It can mess with your liver enzymes, so if you notice yellowing skin, dark urine, or unexplained fatigue, get checked. And if you’re on blood thinners or probenecid, cephalexin can interact, changing how those drugs work. It’s not just about taking the pill—it’s about knowing what your body’s doing underneath.

What you’ll find in the posts below isn’t just a list of side effects. It’s real-world insight from people who’ve been there, plus clinical facts you won’t get from a drug label. You’ll see how cephalexin compares to other antibiotics, what symptoms to track after starting it, and how to tell if it’s working—or if something worse is happening. No fluff. No marketing. Just what you need to stay safe while your body fights the infection.