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 works by stopping bacteria from building cell walls, which kills them or stops them from spreading. But like all antibiotics, it doesn’t just target bad bacteria—it can mess with your gut, skin, and even your immune response. Not everyone gets side effects, but if you do, knowing what’s normal and what’s dangerous can make all the difference.

One of the most common issues is gastrointestinal upset, including nausea, vomiting, diarrhea, and stomach cramps. It’s not rare—up to 1 in 5 people report it. If you get mild diarrhea, it might just be your gut adjusting. But if it turns watery, bloody, or lasts more than a few days, it could be Clostridioides difficile, a dangerous gut infection triggered by antibiotics wiping out good bacteria. That’s not something to ignore. Then there’s the risk of allergic reactions, especially if you’re allergic to penicillin or other beta-lactam antibiotics. Rash, itching, swelling, or trouble breathing? Get help fast. These aren’t just inconveniences—they can be life-threatening.

Some people feel dizzy or tired while taking cephalexin. That’s usually mild, but if it hits hard, don’t drive or operate machinery. Long-term use can also throw off your yeast balance, leading to vaginal yeast infections or oral thrush. And while rare, cephalexin can affect kidney function, especially in older adults or those with existing kidney issues. That’s why dosing matters—your doctor doesn’t just pick a number out of thin air.

What you won’t find in the brochure? Real stories from people who had to stop because of a rash that spread, or the diarrhea that lasted three weeks. That’s why the posts below dig into what actually happens when people take cephalexin—not just the textbook list, but the messy, personal, sometimes scary details. You’ll see how side effects vary by age, dosage, and other meds you’re on. You’ll learn when to push back on your doctor, when to wait it out, and when to head to the ER. This isn’t guesswork. It’s what people actually experienced—and what you need to know before you start the bottle.