The most common side effects of cephalexin 500mg are digestive problems: nausea, diarrhea, and stomach pain. These affect roughly 1 in 10 people who take the drug. Most are mild and tend to ease as your body adjusts over the first few days of treatment. Serious reactions are rare, occurring in fewer than 1 in 1,000 people.
Common Digestive Side Effects
Cephalexin works by killing bacteria, but it doesn’t distinguish between the harmful bacteria causing your infection and the helpful bacteria living in your gut. That disruption is why digestive symptoms are by far the most frequent complaint. Diarrhea is the single most commonly reported side effect. Nausea, stomach cramps, bloating, and heartburn also show up regularly.
Other digestive effects that some people notice include acid stomach, belching, dry mouth, and general stomach upset. These are considered nuisance-level side effects. They often improve within a day or two as your body adjusts, and they typically resolve once you finish your course. Taking the capsule with food or a full glass of water can help reduce nausea and stomach irritation.
Yeast Infections During Treatment
Because cephalexin disrupts the natural balance of microorganisms in your body, it can allow yeast to overgrow. This commonly shows up as a vaginal yeast infection or, less often, oral thrush (white patches in the mouth). Research published in the American Journal of Obstetrics & Gynecology found that people who used antibiotics in the prior month were about 75% more likely to develop a vaginal yeast infection compared to those who hadn’t taken antibiotics. The risk increases with longer courses of treatment and is higher if you’ve had yeast infections before.
If you notice itching, unusual discharge, or white patches in your mouth while taking cephalexin, an over-the-counter antifungal treatment is usually enough to manage it.
Risk of C. diff Infection
A more concerning gut-related complication is infection with a bacterium called C. diff, which can take hold when antibiotics wipe out the protective bacteria in your intestines. You’re up to 10 times more likely to develop a C. diff infection while taking an antibiotic and in the month after finishing one, and longer courses of antibiotics can roughly double that risk further.
The key warning sign is diarrhea that is watery, severe, or contains blood or mucus. This is different from the ordinary loose stools that antibiotics sometimes cause. C. diff diarrhea tends to be persistent and worsening rather than mild and stable. If your diarrhea becomes severe or bloody at any point during or after your course, that warrants prompt medical attention. The effects of antibiotics on your gut bacteria can linger for several months, so a C. diff infection can appear even weeks after you stop taking cephalexin.
Allergic Reactions
Cephalexin belongs to the cephalosporin family of antibiotics, which is chemically related to penicillin. If you have a known penicillin allergy, there is a 1% to 8% chance of cross-reactivity with first-generation cephalosporins like cephalexin. That means most people with a penicillin allergy can take it safely, but it’s not a zero risk.
Mild allergic reactions can include a skin rash or itching. More serious allergic reactions, though rare, can involve swelling of the face, lips, or throat, difficulty breathing, or a rapid heartbeat. These symptoms typically appear within the first few doses. However, certain serious skin reactions can develop weeks to even months after starting the medication, so a delayed rash shouldn’t be dismissed just because you’ve been on the drug for a while.
Signs That Need Immediate Attention
Serious side effects from cephalexin occur in fewer than 1 in 1,000 people, but knowing what to watch for matters. The red flags include:
- Severe or bloody diarrhea, which could signal C. diff or intestinal inflammation
- Clay-colored or unusually light stools, which can indicate a liver problem
- Blood in urine or dark/tarry stools, which may reflect a blood-related complication
- Vomiting blood
- Swelling, hives, or difficulty breathing, which suggest a serious allergic reaction
How Dosing Relates to Side Effects
The standard adult dose of cephalexin is 250mg every 6 hours, though 500mg every 12 hours is a common alternative. For more severe infections, the daily total can go as high as 4 grams, split into two to four doses throughout the day. While the available clinical data doesn’t clearly show that side effects scale with dose, higher daily totals mean more antibiotic reaching your gut, which logically increases the chance of digestive disruption. If you’re taking 500mg capsules multiple times a day for a more serious infection, you may notice more stomach-related symptoms than someone on a lower total dose.
People with reduced kidney function clear cephalexin from their body more slowly, which effectively increases how much drug is circulating at any given time. This can amplify side effects. If you have kidney problems, your prescriber will typically adjust the dose or spacing to account for this.
What to Expect Over the Course of Treatment
Most people who experience side effects notice them within the first day or two of starting cephalexin. Nausea and mild diarrhea are usually worst at the beginning and improve as treatment continues. A typical course of cephalexin runs 7 to 14 days, and most side effects resolve within a few days of finishing.
The exceptions are gut-related complications like C. diff, which can appear during treatment or up to several weeks afterward, and serious skin reactions, which can emerge weeks to months after starting the medication. Yeast infections also tend to show up later in a course or shortly after finishing. So even after your last dose, it’s worth staying aware of unusual symptoms for a few weeks.