Cephalexin is a first-generation cephalosporin antibiotic commonly prescribed for skin infections, urinary tract infections, ear infections, and strep throat. Its most frequent side effect is diarrhea, followed by other digestive symptoms like nausea, vomiting, and stomach pain. Most people tolerate it well, but there are some less common reactions worth knowing about before you start a course.
Digestive Side Effects
Stomach and intestinal issues are by far the most common complaints. Diarrhea tops the list in clinical trials, and it can range from mildly loose stools to more frequent watery bowel movements. Nausea, vomiting, heartburn, and abdominal pain also show up regularly. These symptoms tend to be mild and improve on their own as your body adjusts or once you finish your prescription.
Taking cephalexin with food can help reduce nausea and stomach upset without significantly affecting how well the drug works. If digestive symptoms are bothering you, try having a small meal or snack alongside each dose. Staying hydrated is especially important if you’re dealing with diarrhea.
How Long Side Effects Typically Last
Mild digestive symptoms usually ease within a day or two of finishing your antibiotic course. For most people, the worst of the nausea or loose stools happens in the first few days and then settles down even while still taking the medication. If diarrhea persists or worsens after you stop cephalexin, that’s a different situation (covered below) and worth a call to your doctor.
Yeast Infections and Genital Irritation
Like most antibiotics, cephalexin kills off some of the beneficial bacteria that normally keep yeast in check. This can lead to vaginal yeast infections, vaginal discharge, or genital and anal itching. These effects are less common than digestive issues but not rare, especially during longer courses. Eating probiotic-rich foods like yogurt during treatment may help, though the evidence for this is mixed.
Headache, Dizziness, and Fatigue
Some people report headaches, dizziness, or general tiredness while taking cephalexin. These tend to be mild. In rare cases, the FDA label notes agitation, confusion, and even hallucinations, though these neurological effects are uncommon and more likely to occur in people with reduced kidney function, where the drug can build up to higher-than-intended levels in the body.
Skin Rashes and Allergic Reactions
Rashes can appear during a cephalexin course, and they fall into two broad categories. A mild, non-itchy rash that appears a few days into treatment is relatively common with antibiotics and doesn’t always signal a true allergy. However, a rash that’s accompanied by intense itching, hives, facial or throat swelling, or difficulty breathing points to a genuine allergic reaction that needs immediate medical attention.
Severe skin reactions like widespread blistering or peeling are extremely rare but possible. If your skin starts to blister or you develop painful sores in your mouth or around your eyes, stop the medication and seek emergency care.
Cross-Reactivity With Penicillin Allergy
Cephalexin is structurally related to penicillin, and people with a penicillin allergy sometimes react to cephalosporins as well. For first-generation cephalosporins like cephalexin, the cross-reactivity rate in people with confirmed penicillin allergy falls in the 1% to 8% range. Third-generation cephalosporins carry a lower risk (under 1%). If you have a known penicillin allergy, your prescriber should already be aware, but it’s always worth double-checking.
Antibiotic-Associated Diarrhea and C. Diff
There’s a meaningful difference between the ordinary loose stools antibiotics cause and an infection with Clostridioides difficile, a bacterium that can flourish when antibiotics wipe out competing gut bacteria. C. diff symptoms often begin within 5 to 10 days of starting an antibiotic, but they can appear as late as three months after finishing a course.
Mild C. diff causes watery diarrhea three or more times a day along with belly cramping. Severe cases escalate to 10 to 15 episodes of watery diarrhea per day, fever, significant abdominal pain, blood or pus in the stool, and dehydration. This is a serious complication that requires its own treatment. If you develop persistent watery diarrhea, especially with a fever, during or after a cephalexin course, get it evaluated promptly.
Effects on Blood Cells and Kidneys
Rarely, cephalexin can affect blood cell counts. Reported issues include a drop in white blood cells (which help fight infection), a drop in platelets (which help blood clot), and a type of anemia where red blood cells are destroyed faster than normal. These are uncommon and typically identified through routine blood work rather than symptoms you’d notice at home. Mild, reversible increases in liver enzymes have also been documented.
Reversible kidney inflammation (interstitial nephritis) is another rare but recognized reaction. People with pre-existing kidney problems need adjusted dosing. When kidney function is moderately reduced, the interval between doses is extended. With severe kidney impairment, doses may be spaced as far apart as every 12 to 24 hours to prevent the drug from accumulating to potentially toxic levels.
Interaction With Metformin
If you take metformin for diabetes, cephalexin can raise your metformin levels significantly. Research has shown that cephalexin increases the peak concentration of metformin in the blood by about 54% and its overall exposure by roughly 19%, while reducing the kidneys’ ability to clear metformin by 17%. This happens because both drugs compete for the same transport system in the kidneys.
Higher metformin levels increase the risk of metformin’s own side effects, particularly nausea, diarrhea, and in extreme cases, a dangerous buildup of lactic acid. If you’re on metformin and prescribed cephalexin, your provider may want to monitor your blood sugar more closely or consider an alternative antibiotic.
Side Effects in Children
Children experience largely the same side effects as adults: stomach pain, diarrhea, heartburn, and occasional vomiting. Giving the liquid suspension with food can help settle an upset stomach. Severe skin rashes or itching in a child warrant a prompt call to their pediatrician, as these can indicate an allergic reaction. Watch for signs of dehydration if diarrhea becomes frequent, particularly in younger children who lose fluids more quickly.