cephalexin
Brand names: Keflex
Cephalexin is an antibiotic medicine. It fights bacterial infections in your body.
Drug Pricing (NADAC)
Generic Price
$0.05/unit
Generic Available
Yes (13 manufacturers)
Pricing data from NADAC (CMS), effective December 18, 2024. Compare all drug costs →
What it does
Cephalexin treats infections caused by bacteria.
Common side effects
Diarrhea, Nausea, Vomiting
Key warnings
Cephalexin can cause severe allergic reactions.
How It Works
Cephalexin belongs to a class of drugs called cephalosporins. It works by stopping the growth of bacteria. This helps your body fight off the infection.
How to Take It
Take cephalexin capsules as your doctor tells you. Adults usually take 250 mg every 6 hours, or 500 mg every 12 hours. Children's doses are based on their weight. You should take this medicine for 7 to 14 days.
Pregnancy & Breastfeeding
Tell your doctor if you are pregnant or breastfeeding. Studies haven't shown harm to the baby, but talk to your doctor about the risks and benefits. Cephalexin passes into breast milk, but the amount is very low.
Missed Dose
If you miss a dose, take it as soon as you remember. If it is almost time for your next dose, skip the missed dose and continue with your regular schedule.
Storage
Store cephalexin capsules at room temperature, away from moisture and heat.
Side Effects (from patient reports)
Based on 13,809 FDA adverse event reports.
FDA Adverse Event Report Analysis
Detailed analysis of 20,636 reports from the FDA Adverse Event Reporting System (FAERS). Reports span 2003–2025.
Total Reports
20,636
Death-Related Reports
1,749
Hospitalization Reports
7,230
Top Indication
Product Used For Unknown Indication
Gender Distribution
Age Distribution
Most Reported Adverse Reactions (FAERS)
| # | Reaction | Reports |
|---|---|---|
| 1 | CHRONIC KIDNEY DISEASE | 2,155 |
| 2 | RENAL FAILURE | 1,554 |
| 3 | ACUTE KIDNEY INJURY | 1,538 |
| 4 | PAIN | 1,365 |
| 5 | DIARRHOEA | 1,350 |
| 6 | FATIGUE | 1,239 |
| 7 | NAUSEA | 1,220 |
| 8 | DYSPNOEA | 1,164 |
| 9 | ANXIETY | 1,132 |
| 10 | RASH | 1,093 |
| 11 | HEADACHE | 1,089 |
| 12 | DRUG INEFFECTIVE | 988 |
| 13 | OFF LABEL USE | 854 |
| 14 | VOMITING | 837 |
| 15 | DIZZINESS | 828 |
Reactions in Death Reports
Reactions in Hospitalization Reports
Source: FDA FAERS (Adverse Event Reporting System) FDA FAERS (Adverse Event Reporting System) Reports are voluntary and do not establish causation
Serious Warnings
Cephalexin can cause severe allergic reactions. Tell your doctor if you are allergic to penicillin or other cephalosporin antibiotics. This medicine can also cause severe diarrhea. If you get diarrhea, tell your doctor right away. People with kidney problems may need a lower dose to prevent seizures.
Known Drug Interactions
( 7.1 ) Probenecid-The renal excretion of cephalexin is inhibited by probenecid. Co-administration of probenecid with cephalexin is not recommended. 7.2 Probenecid The renal excretion of cephalexin is inhibited by probenecid.
Mechanism: Probenecid blocks your kidneys from removing cephalexin from your body. This causes the antibiotic to build up to higher levels than intended.
What to do: You should not take these two medications together unless specifically directed by your doctor.
7 DRUG INTERACTIONS Metformin: increased metformin concentrations. ( 7.3 ) 7.1 Metformin Administration of cephalexin with metformin results in increased plasma metformin concentrations and decreased renal clearance of metformin. Careful patient monitoring and dose adjustment of metformin is recommended in patients concomitantly taking cephalexin and metformin [see Clinical Pharmacology (12.3) ].
Mechanism: Cephalexin makes it harder for your kidneys to remove metformin from your body. This causes the metformin to stay in your blood longer and reach higher levels than usual.
What to do: Your doctor should monitor you carefully and may need to lower your metformin dose while you are taking the antibiotic. Watch for signs of low blood sugar or other side effects during treatment.
Common Questions
Can I take cephalexin if I'm allergic to penicillin?
Can cephalexin cause diarrhea?
How long should I take cephalexin?
What should I do if I miss a dose?
Can I take cephalexin with food?
Does cephalexin interact with other medicines?
Can cephalexin affect my blood sugar?
Can cephalexin cause a false positive urine glucose test?
What if I still feel sick after taking cephalexin for a few days?
Can I drink alcohol while taking cephalexin?
What are the common side effects of cephalexin?
Does cephalexin interact with other medications?
What drug class is cephalexin?
Is cephalexin safe during pregnancy?
Has cephalexin been recalled?
Active Recalls
Failed Impurities/Degradation Specifications An out-of-specification result was observed in the related substance test at the sixth month of stability analysis. The individual impurity was identified to be Cephalexin Glucose Adduct.
Ascend Laboratories, LLC
Labeling: Not Elsewhere Classified: Back label states Each contains: cephalexin monohydrate, USP equivalent to 2.5g' instead of Each Bottle contains: cephalexin monohydrate, USP equivalent to 5g
Bryant Ranch Prepack, Inc.
Labeling: Not Elsewhere Classified: Back Label states Each contains: cephalexin monohydrate, USP equivalent to 5g' on the back label instead of Each Bottle contains: cephalexin monohydrate, USP equivalent to 5g'
Bryant Ranch Prepack, Inc.
Failed Impurities/degradation specifications: Repackager recall due to Out of Specification detected by manufacturer for Individual Unidentified Impurity found during related substance test analysis of Cephalexin
Preferred Pharmaceuticals, Inc.
Related Medications in First-Generation Cephalosporin
Other drugs grouped near cephalexin — same-class peers and common alternatives.
amikacin
Amikin
Amikacin is an antibiotic medicine.
Compare with cephalexin →
amoxicillin
Amoxil
Amoxicillin and Clavulanate Potassium is a combination medicine used to fight bacterial infections.
Compare with cephalexin →
amoxicillin/clavulanate
Augmentin
Augmentin is a combination of two medicines, amoxicillin and clavulanate.
Compare with cephalexin →
ampicillin/sulbactam
Unasyn
Unasyn is a combination of two antibiotics that fights bacteria in your body.
Compare with cephalexin →
azithromycin
Zithromax, Z-Pack
Azithromycin is an antibiotic that fights bacteria.
Compare with cephalexin →
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Common Drug Interactions
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Related Health & Safety Data
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What the FDA Data Shows for cephalexin
The FDA label for cephalexin (sold under brand names such as Keflex) classifies it as a prescription-only medication in the First-Generation Cephalosporin class. Cephalexin treats infections caused by bacteria. Official labeling lists 5 commonly reported side effects, including Diarrhea, Nausea, Vomiting.
Post-market surveillance from the FDA Adverse Event Reporting System (FAERS) captures real-world experience. For this drug, FAERS contains 13,809 voluntary reports. The database also lists 2 documented drug interactions derived from FDA labeling, with the top-flagged interaction rated moderate severity. NADAC pricing from CMS shows a generic unit cost of $0.05.
Report counts do not establish causation — a FAERS entry documents a temporal association, not proof that the drug produced the outcome. Widely prescribed medications naturally accumulate more reports than niche therapies, so raw totals must be interpreted alongside total exposure. Shortage status, recall history (currently 4 recall records on file), and patent information further shape supply and switching decisions. This page summarizes public FDA data for educational purposes only and is not a substitute for professional medical advice — always consult a licensed healthcare provider before starting, stopping, or changing any medication.
Data Sources
Drug labeling: FDA Drug Labels (SPL/DailyMed). Adverse events: FDA Adverse Event Reporting System (FAERS). Pricing: CMS National Average Drug Acquisition Cost (NADAC).
FAERS reports are voluntary and do not establish causation. Drug interactions are derived from FDA labeling and clinical references. Always consult a healthcare professional before making medication decisions.
Last updated: February 11, 2025
Read our methodology — how this data is sourced, computed, and verified.
All federal data sources used on this page
- FDA Orange Book — approved drug products with therapeutic equivalence. accessdata.fda.gov/cder/ob
- FDA DailyMed — NIH-hosted drug labeling for FDA-approved meds. dailymed.nlm.nih.gov
- FDA Adverse Event Reporting System (FAERS) — post-marketing safety surveillance. fda.gov/drugs/faers
- NLM RxNorm — standardized clinical drug nomenclature. nlm.nih.gov/research/umls/rxnorm
- CMS Medicare Part B Drug Average Sales Price Files — federal drug pricing data. cms.gov/medicare/part-b-drugs/asp
- FDA Drug Shortages Database — current and resolved drug shortage tracking. accessdata.fda.gov/drugshortages