Cephalexin 500 mg is an antibiotic used to treat bacterial infections of the skin, urinary tract, respiratory system, bones, and ears. It belongs to the cephalosporin family, a group of antibiotics related to penicillin, and works by breaking down the cell walls of bacteria so they can’t survive and multiply. The 500 mg capsule is the most commonly prescribed strength for adults.
Conditions Cephalexin Treats
Cephalexin is a broad first-line antibiotic, meaning doctors often reach for it early when they suspect a straightforward bacterial infection. Its approved uses span several parts of the body:
- Skin and soft tissue infections: Cellulitis, wound infections, and abscesses caused by common skin bacteria like staph and strep. This is one of the most frequent reasons it’s prescribed.
- Urinary tract infections (UTIs): Bladder infections (cystitis) and, in some cases, kidney infections. A typical course for an uncomplicated UTI runs 5 to 7 days.
- Upper respiratory infections: Strep throat, tonsillitis, and middle ear infections (otitis media), particularly in cases where a confirmed or strongly suspected bacterial cause is present.
- Lower respiratory infections: Some forms of bacterial pneumonia and bronchitis.
- Bone infections: Osteomyelitis, often as a follow-up oral antibiotic after initial hospital treatment.
Cephalexin does not work against viruses. It won’t help with colds, the flu, or COVID. It’s also not effective against MRSA, a resistant strain of staph bacteria, which is an important distinction when skin infections don’t respond to treatment.
How You’ll Typically Take It
Most adults are prescribed 500 mg two to four times daily, depending on the type and severity of the infection. Your doctor determines the frequency and length of the course based on what’s being treated. Skin infections like cellulitis often require 7 to 14 days, while uncomplicated UTIs typically need 5 to 7 days.
You can take cephalexin with or without food. Eating a meal slightly delays the time it takes for the drug to reach peak levels in your blood, but doesn’t change the total amount your body absorbs. If the medication bothers your stomach, taking it with food or a snack can help.
Finishing the full course matters, even if you feel better after a few days. Stopping early gives surviving bacteria a chance to regrow, potentially leading to a harder-to-treat infection.
Common Side Effects
Diarrhea is the most frequently reported side effect in clinical trials. Nausea, vomiting, stomach pain, and indigestion also occur. These digestive issues are generally mild and tend to resolve once you finish the course. Eating before taking your dose can reduce the stomach upset.
Yeast infections are another known side effect, particularly vaginal yeast infections. Antibiotics disrupt the natural balance of bacteria in the body, which can allow yeast to overgrow. Oral thrush (a yeast infection in the mouth) is also possible.
Allergic Reactions and Penicillin Sensitivity
Because cephalexin is chemically related to penicillin, people with a penicillin allergy sometimes wonder whether it’s safe. The cross-reactivity rate for first-generation cephalosporins like cephalexin falls in the 1% to 8% range among people with confirmed penicillin allergies. That risk is real but relatively low. For comparison, newer third-generation cephalosporins carry a cross-reactivity rate below 1%.
Allergic reactions can range from a mild rash or hives to rare but serious responses like severe swelling or anaphylaxis. If you develop a rash, facial swelling, or difficulty breathing while taking cephalexin, seek immediate medical attention. If you’ve had a severe allergic reaction to any penicillin-type antibiotic in the past, make sure your prescriber knows before starting treatment.
Drug Interactions to Know About
Cephalexin has a relatively short list of significant drug interactions, but a few are worth flagging. Probenecid, a gout medication, slows the rate at which your kidneys clear cephalexin from your body, effectively increasing the antibiotic’s levels in your bloodstream. Metformin, the widely used diabetes medication, can also interact. Certain diuretics (medications that increase urine output) may affect how the drug is processed. Let your prescriber know about any medications you’re taking, including other antibiotics.
Safety During Pregnancy and Breastfeeding
Cephalexin is considered one of the safer antibiotic options during pregnancy. Untreated infections carry their own risks to both mother and baby, and cephalexin is frequently chosen when treatment is necessary.
Only tiny amounts of the drug pass into breast milk, so it’s generally safe while breastfeeding. In rare cases, nursing infants have developed diarrhea or signs of oral thrush. If your baby stops feeding well, develops loose stools, or shows white patches in the mouth, those are signs worth mentioning to a healthcare provider. There’s no evidence that cephalexin affects fertility in men or women.
What Cephalexin Won’t Do
Cephalexin targets a specific range of bacteria. It’s not a broad-spectrum powerhouse, and it won’t cover every infection. Bacteria that are resistant to first-generation cephalosporins, including MRSA and many gram-negative bacteria that cause complicated urinary or abdominal infections, require different antibiotics. If your symptoms aren’t improving after two to three days on cephalexin, that’s a signal your infection may involve bacteria the drug can’t reach.