Cephalexin side effects: what to expect and when to call your doctor

If your doctor prescribed cephalexin, you probably want to know the downside. This short guide explains the common, mild reactions you may see, the rare but serious problems to watch for, and simple steps to reduce risk. Read this so you can spot trouble fast and feel confident while taking the drug.

Common, mild side effects

Most people tolerate cephalexin well. The mild effects usually go away on their own. Expect things like diarrhea, nausea, stomach pain, and sometimes a headache or dizziness. Some people get a yeast infection (oral or vaginal) after antibiotics change the natural bacteria. A mild, itchy rash can happen too.

Tips to handle mild symptoms: take cephalexin with food if your stomach hurts, drink plenty of water, and consider a probiotic or plain yogurt to reduce diarrhea risk. If diarrhea is tolerable and not bloody, it often resolves after the course ends. If it gets worse, read the next section.

Serious reactions and when to seek help

Stop the drug and get medical help right away for signs of a severe allergic reaction: sudden rash, hives, swelling of the face or throat, trouble breathing, or fainting. Those could be anaphylaxis and need emergency care.

Other red flags: very bad belly pain, persistent watery or bloody diarrhea (could be C. difficile), yellowing of the skin or eyes, dark urine, high fever, or a rash that blisters and spreads. These suggest a serious infection, liver problem, or a severe skin reaction and require prompt medical attention.

People with kidney disease need a lower dose because cephalexin is cleared by the kidneys. If you have kidney problems, tell your doctor before starting the antibiotic.

Also be cautious if you have a penicillin allergy. Cross-reaction between penicillins and cephalosporins is possible but not guaranteed. Tell your prescriber about any past allergic reactions to antibiotics.

Drug interactions to note: probenecid can raise cephalexin levels. Some antibiotics can affect how well hormonal birth control works, though cephalosporins are less likely to do this than others. If you take blood thinners like warfarin, mention it—antibiotics can sometimes change bleeding risk.

How long do side effects last? Mild issues often stop a few days after finishing the antibiotic. Serious problems can appear during or after treatment, so stay alert for new symptoms even after the last dose.

If you aren’t sure what to do, call your doctor or a pharmacist. Describe your symptoms and how long you’ve been on cephalexin. Quick action prevents small problems from becoming big ones.

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