Cephalexin: Nursing Drug Guide, Beta-Lactam Allergy & NCLEX Review
Cephalexin is a first-generation cephalosporin—not a free pass around penicillin allergy. Before the first dose, verify beta-lactam history, adjust dosing when creatinine clearance is below 30 mL/min, and stop therapy at the first sign of urticaria, angioedema, or hypotension.
Serious hypersensitivity reactions including anaphylaxis, angioedema, and Stevens-Johnson syndrome have been reported with cephalexin. Cephalexin is contraindicated in patients with known hypersensitivity to cephalexin or other cephalosporins. Prescribing information notes cross-hypersensitivity among beta-lactams may occur in up to 10% of patients with penicillin allergy—do not assume cephalexin is safe because it is not a penicillin. Before therapy, inquire about reactions to cephalosporins, penicillins, and other drugs. If allergy signs develop, discontinue cephalexin and treat per facility protocol.
📋 Contents
⚡ Quick facts
💡 Key takeaway
Never give cephalexin until cephalexin, cephalosporin, and penicillin allergy history is verified—“not a penicillin” does not mean allergy-free. When creatinine clearance is below 30 mL/min, confirm pharmacy renal adjustment before the standard 250–500 mg schedule. At the first sign of urticaria, angioedema, wheeze, or hypotension, hold the dose and escalate before any further beta-lactam administration.
Most common brand names
Cephalexin is available as generic capsules (250 mg and 500 mg) and oral suspension (125 mg/5 mL and 250 mg/5 mL per labeling). Confirm formulation, concentration for suspensions, and that the patient can take oral therapy.
Common brand names include Keflex. Do not confuse with other cephalosporins (e.g., cefdinir, cefuroxime) or penicillins on the MAR—verify the generic name on every administration pass.
Why we give it — Indications
Cephalexin is a first-generation cephalosporin indicated for susceptible respiratory tract, otitis media, skin and skin structure, bone, and genitourinary infections per FDA-approved labeling. Nurses commonly administer it for cellulitis, uncomplicated urinary tract infection, streptococcal tonsillitis or pharyngitis when ordered, and other community infections when culture or local epidemiology supports therapy.
| Use | Detail |
|---|---|
| Respiratory tract | Treatment of respiratory tract infections caused by susceptible Streptococcus pneumoniae and Streptococcus pyogenes per labeling. |
| Otitis media | Indicated for otitis media caused by susceptible organisms including S. pneumoniae, H. influenzae, S. aureus, S. pyogenes, and M. catarrhalis. |
| Skin and skin structure | Used for susceptible Gram-positive skin infections including S. aureus and S. pyogenes; coordinate with wound assessment and cultures. |
| Bone infections | Indicated for bone infections caused by susceptible S. aureus and Proteus mirabilis. |
| Genitourinary tract | Includes acute prostatitis and UTIs caused by susceptible E. coli, P. mirabilis, and K. pneumoniae. |
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Use only for infections proven or strongly suspected to be bacterial to limit resistance; methicillin-resistant S. aureus and most enterococci are resistant per labeling.
How it works
Cephalexin is a bactericidal cephalosporin that inhibits bacterial cell-wall synthesis. It is acid stable and may be given without regard to meals per labeling. More than 90% of unchanged drug is excreted in urine within 8 hours—renal impairment prolongs exposure and increases seizure and toxicity risk if doses are not reduced.
Dosing overview
Adults and patients at least 15 years old: usual dose is 250 mg every 6 hours, or 500 mg every 12 hours for streptococcal pharyngitis, skin infections, and uncomplicated cystitis in patients over 15 years. Daily dose ranges from 1 to 4 g in divided doses; more severe infections may need up to 4 g/day. If daily doses above 4 g are required, parenteral cephalosporins should be considered per labeling.
Pediatrics (over 1 year): Usual total daily dose 25–50 mg/kg/day in divided doses for 7–14 days; otitis media 75–100 mg/kg/day divided; severe infections 50–100 mg/kg/day. β-hemolytic streptococcal infections: at least 10 days per labeling.
Duration: Therapy is generally 7–14 days depending on infection type and severity; cystitis should be continued 7–14 days per labeling.
Missed dose: Not specified in the reviewed prescribing information. Do not double doses; contact prescriber or pharmacist for interval guidance.
Onset, peak, duration, and half-life
| Parameter | Value | Nursing relevance |
|---|---|---|
| Absorption | Acid stable; may give without regard to meals | Peak serum ~9, 18, and 32 mcg/mL at 1 h after 250, 500, and 1 g doses |
| Distribution | ~10–15% protein bound | Not specified in the reviewed prescribing information for tissue penetration targets at bedside |
| Excretion | >90% unchanged in urine within 8 h | Review eGFR and creatinine before standard dosing when renal impairment is possible |
| Detectable serum | Up to 6 h after dose (detection 0.2 mcg/mL) | Missed doses and interval errors reduce effective coverage—document administration times |
| Seizure risk | Increased when dose not reduced in renal impairment | Hold and clarify MAR when CrCl <30 mL/min without pharmacy adjustment |
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Before you give it — Safety check
Pretreatment checks
- Allergy history: cephalexin, cephalosporins, penicillins, and other beta-lactams; ask about anaphylaxis, angioedema, Stevens-Johnson syndrome, and timing of prior reactions
- Complete medication reconciliation for metformin, probenecid, and duplicate oral antibiotics
- Review serum creatinine and calculated clearance when renal impairment is possible; maximum daily dose should not exceed 1 g when CrCl is 30–59 mL/min
- Confirm bacterial indication; viral illness with fever alone does not justify empiric antibiotics without prescriber plan
- Verify MAR generic name—cephalexin is often confused with other cephalosporins and penicillins
Contraindications
- Known hypersensitivity to cephalexin or other members of the cephalosporin class
- Active serious hypersensitivity during therapy—discontinue and institute supportive measures per labeling
Important interactions
| Drug / class | Effect | Nursing action |
|---|---|---|
| Metformin | Increased plasma metformin; decreased renal clearance of metformin | Monitor for hypoglycemia; notify pharmacist when either drug starts or stops |
| Probenecid | Inhibits renal excretion of cephalexin—co-administration not recommended | Alert pharmacist if probenecid is on the medication list |
| Anticoagulant therapy | Cephalosporins may prolong prothrombin time in at-risk patients | Monitor INR/bleeding in renal/hepatic impairment, malnutrition, or prolonged courses per labeling |
| Urine glucose testing | False-positive glucose with Benedict’s or Fehling’s solution | Use glucose tests not affected by this interaction when clinically relevant |
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Administration
Route: Oral capsules (250 mg and 500 mg) and oral suspension (125 mg/5 mL and 250 mg/5 mL). Reconstitute suspension at dispensing; refrigerate after mixing; may keep 14 days per labeling.
- Follow medication administration rights; verify patient identity, drug, dose, route, time, and allergy band
- May administer without regard to meals per labeling
- Shake suspension well; measure with oral syringe or calibrated device—not household spoons
- Document exact time given to support q6h/q12h intervals and culture follow-up
Expected therapeutic response
- Defervescence and improving symptoms of bacterial infection (e.g., decreasing purulent drainage, improving dysuria, reduced sore throat severity) after 48–72 hours
- Stable or improving vital signs without new allergic findings
- Culture or clinical follow-up supports continued therapy when ordered for longer courses
Lack of improvement after 72 hours may indicate resistant organisms or nonbacterial illness—notify prescriber rather than assuming nonadherence alone.
Adverse effects
| Adverse effect | Frequency / severity | Nursing response |
|---|---|---|
| Diarrhea, nausea, vomiting, dyspepsia, abdominal pain | Most frequent in clinical trials | Supportive care; hold and escalate if severe, bloody, or with systemic allergy signs |
| Rash / anaphylaxis / angioedema | Serious hypersensitivity reported | Discontinue cephalexin; treat per protocol; update allergy record |
| C. difficile-associated diarrhea | Reported with nearly all antibacterials including cephalexin | Hold antibiotic; contact prescriber; evaluate for antibiotic-associated diarrhea per protocol |
| Drug-induced hemolytic anemia | Acute intravascular hemolysis reported; positive direct Coombs test | If anemia develops during therapy, discontinue and evaluate per labeling |
| Seizures | Reported especially when dose not reduced in renal impairment | Discontinue cephalexin; anticonvulsant therapy if clinically indicated |
| Prolonged prothrombin time | Associated with cephalosporins in at-risk patients | Monitor INR/bleeding when on anticoagulants or prolonged therapy |
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Overdose and toxicity
Symptoms of oral overdose may include nausea, vomiting, epigastric distress, diarrhea, and hematuria per labeling. In the event of overdose, institute general supportive measures. Forced diuresis, peritoneal dialysis, hemodialysis, or charcoal hemoperfusion have not been established as beneficial for cephalexin overdose.
Antidote
No specific antidote is listed in the reviewed prescribing information. Management is supportive. Contact your facility’s poison control or medical toxicology service per local protocol for large ingestions or protracted symptoms.
Contact your facility’s poison control or medical toxicology service per local protocol for large ingestions, protracted vomiting, or uncertainty about total dose taken.
Look-alike / sound-alike and error prevention
- Cephalexin vs cefdinir vs cefuroxime — all cephalosporins but different generation, spectrum, and dosing; verify generic name on MAR
- Cephalexin vs penicillins — nurses may assume cephalexin is safe with penicillin allergy; requires prescriber/pharmacy clearance
- Keflex vs Kefzol (cefazolin) — sound-alike brand names; IV cefazolin is not interchangeable with oral cephalexin
- 250 mg vs 500 mg — standard schedule given without renal adjustment when CrCl <30 mL/min is a common error
- Suspension concentration — 125 mg/5 mL vs 250 mg/5 mL doubles the dose if the wrong bottle is used
Practical bedside notes
| Topic | Bedside guidance |
|---|---|
| Allergy interview | Ask what happened (hives, lip swelling, wheeze, hypotension), when, and with which drug; penicillin rash in childhood still needs prescriber/pharmacy input before cephalexin |
| First-dose watch | Stay available after the first dose when beta-lactam allergy history is uncertain |
| “Not a penicillin” trap | Cephalexin is still a beta-lactam—cross-reactivity up to ~10% with penicillin allergy per labeling |
| Diarrhea teaching | Watery or bloody stools during or after therapy may be C. difficile—escalate; do not mask with antidiarrheals alone without prescriber guidance |
| Renal dosing | When CrCl <30 mL/min, standard q6h dosing is usually wrong—verify pharmacy-adjusted MAR |
| Ask pharmacy when | Unclear allergy history, metformin co-therapy, probenecid on profile, pregnancy/lactation questions, or new rash on therapy |
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Red flags — Stop and act
Beta-lactam hypersensitivity can progress rapidly. Hold cephalexin and escalate immediately when any of the following appear.
- Anaphylaxis symptoms: urticaria, angioedema, bronchospasm, stridor, hypotension, or syncope after a dose
- Generalized rash with mucosal involvement, blistering, or target lesions (possible Stevens-Johnson syndrome / toxic epidermal necrolysis)
- Watery, bloody diarrhea with abdominal cramping—suspect C. difficile-associated diarrhea (can occur during or up to 2 months after antibiotics)
- New anemia, hemoglobinuria, or falling hemoglobin during therapy—evaluate for drug-induced hemolytic anemia
- Seizure activity in renal impairment—discontinue and notify prescriber; dosing may have been excessive
High-risk populations
| Population | Considerations |
|---|---|
| Penicillin allergy history | Cross-hypersensitivity may occur in up to 10%—do not administer without allergy/pharmacy clearance |
| Prior cephalosporin reaction | Contraindicated if serious hypersensitivity to cephalexin or cephalosporins |
| Renal impairment (CrCl <30) | Reduced dosing required; seizure risk if dose not reduced; monitor longer for toxicity per labeling |
| Elderly patients | Greater likelihood of decreased renal function—careful dose selection |
| Patients on metformin | Increased metformin levels—monitor for hypoglycemia |
| Pregnancy | Available epidemiologic data have not established increased risk of major birth defects; studies have limitations |
| Lactation | Present in human milk; relative infant dose <1% of maternal weight-adjusted dose; effects on infant not established |
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Monitoring and documentation
Monitor
- Allergy signs for at least the first dose and whenever therapy restarts after interruption
- GI tolerance: diarrhea frequency, blood in stool, abdominal pain
- Clinical response to infection (temperature, pain, wound appearance, dysuria resolution)
- Prothrombin time / bleeding in at-risk patients on anticoagulants or prolonged therapy
- Renal function and signs of toxicity when CrCl <30 mL/min or daily dose approaches 4 g
- Blood glucose in patients on metformin
Document
- Allergy verification source (patient, family, chart, allergy service)
- Dose, route, time, and indication; renal dose adjustments confirmed with pharmacy
- Any reaction with onset time, interventions, and prescriber notification
Patient teaching
- Take the full course unless the prescriber stops early—stopping early can fail treatment and promote resistance
- Report rash, hives, lip or tongue swelling, trouble breathing, or dizziness immediately—do not take another dose first
- Expect mild diarrhea sometimes; report watery or bloody stools, severe abdominal pain, or fever
- Tell clinicians about all medicines including metformin; ask before starting new prescriptions
- Finish the course unless an allergic reaction or severe diarrhea occurs—then call before taking another dose
- Refrigerate liquid cephalexin if dispensed as suspension; shake well before each dose
The Hold Rule
- Known hypersensitivity to cephalexin or cephalosporins
- New urticaria, angioedema, wheeze, hypotension, or other anaphylaxis symptoms after any dose
- Standard adult dosing (e.g., 250 mg q6h or 500 mg q12h) when creatinine clearance is <30 mL/min without pharmacy renal adjustment
- Watery or bloody diarrhea suggesting C. difficile-associated diarrhea
- Developing anemia during therapy until drug-induced hemolysis is evaluated
- Unclear penicillin/cephalosporin allergy documentation—clarify with pharmacist or prescriber before first dose
Hold parameters may vary by institutional protocol. Follow prescriber orders, pharmacy guidance, and facility policy.
Clinical practice integration and workflow
Cephalexin is a workhorse oral antibiotic—and a frequent source of “safe because it is not penicillin” errors. Build beta-lactam allergy verification and renal dose checks into every administration pass.
1. Check-before-you-give protocol
- Right patient, drug, dose, route, time—and verified cephalexin/cephalosporin/penicillin allergy status
- Read allergy band and chart; reconcile home antibiotics and recent courses
- Confirm renal-adjusted dose when creatinine clearance is <30 mL/min
- Verify suspension concentration and reconstitution date if liquid formulation
2. High-alert and safety badge
Beta-lactam allergy risk — treat first doses as high-stakesAnaphylaxis and serious skin reactions are reported with cephalexin. Cross-reactivity with penicillin allergy is possible. Know unit anaphylaxis response per facility protocol.
3. Clinical workflow: hold and question rules
- Stop the dose when any systemic allergy sign appears—do not restart after urticaria or wheeze
- Question duplicate oral antibiotics on the MAR and empiric orders without culture indication
- Escalate lack of clinical improvement after 48–72 hours for culture review and alternate therapy
4. Critical teach-back questions
- “What allergy symptoms should you report right away?” (Hives, swelling, breathing trouble, dizziness.)
- “What diarrhea should you report during antibiotics?” (Watery or bloody stools, severe cramping—possible C. difficile.)
5. Care coordination
Pharmacist: Penicillin–cephalosporin allergy clarification, renal dose adjustment, metformin interaction monitoring, and antibiotic substitution when beta-lactam allergy is confirmed.
Prescriber / allergy service: Notify for systemic reaction, hemolytic anemia work-up, seizures in renal impairment, or need for alternate antibiotic class.
🧠 Quick mental checklist
- Is beta-lactam allergy history verified—not dismissed because cephalexin is a cephalosporin?
- Any lip swelling, hives, wheeze, or hypotension after the last dose?
- Is creatinine clearance <30 and is standard q6h dosing still on the MAR?
- Is metformin co-therapy flagged for hypoglycemia monitoring?
- Watery or bloody diarrhea developing during or after therapy?
Cephalexin NCLEX practice questions
Practice NCLEX-style clinical judgment practice for cephalexin with a tabbed case (MAR, labs, history, nursing notes), then priority action, cue recognition (SATA), trend interpretation, matrix urgency matching, penicillin-allergy judgment, and documentation cloze—recognise cues → analyse → prioritise → act → evaluate outcomes.
Select a tab to view MAR, labs, history, and nursing note details for this case.
- Cephalexin 500 mg PO q6h scheduled — 0800 dose given; 1400 dose due
- Metformin 1000 mg PO BID with meals — continued
- Acetaminophen 650 mg PO q6h PRN pain — none given today
- Pharmacy note on MAR: “renal dose pending—CrCl 22” (no adjustment entered yet)
- Creatinine 2.1 mg/dL (baseline 1.9); eGFR 22 mL/min/1.73 m²
- WBC 10.8 ×10⁹/L; temp 37.9 °C
- Hemoglobin 11.4 g/dL (stable from admission)
- Wound culture from leg: methicillin-susceptible Staphylococcus aureus
- 72-year-old admitted for lower-leg cellulitis; erythema spreading despite 24 h of oral therapy at home
- Allergy band: “penicillin—rash as a child” (no anaphylaxis documented)
- Type 2 diabetes on metformin; chronic kidney disease stage 4
- Prescriber ordered cephalexin because “patient is not on penicillin”
- 0810: 0800 cephalexin given; patient reports mild nausea—no rash
- 1335: New pruritic wheals on chest; BP 96/54; faint expiratory wheeze; patient reports throat tightness
- 1340: Nurse held 1400 cephalexin; initiated anaphylaxis pathway per unit protocol; notified prescriber
Answer key & rationale
Frequently asked questions
What must I check before giving cephalexin?
Verify allergy history for cephalexin, cephalosporins, penicillins, and other beta-lactams; clarify vague childhood penicillin rash with the prescriber or pharmacist. Confirm bacterial indication, review creatinine clearance when renal impairment is possible, reconcile metformin and probenecid, and verify the correct generic name on the MAR.
When should a nurse hold cephalexin?
Hold for known hypersensitivity to cephalexin or cephalosporins, new urticaria, angioedema, wheeze, or hypotension after a dose, watery or bloody diarrhea suggesting C. difficile, standard dosing when creatinine clearance is below 30 mL/min without pharmacy adjustment, or unclear allergy documentation.
Can patients with penicillin allergy receive cephalexin?
Cross-hypersensitivity among beta-lactams may occur in up to 10% of patients with penicillin allergy per prescribing information. Cephalexin is not automatically safe because it is a cephalosporin—obtain prescriber or pharmacist/allergy guidance before the first dose.
What renal dose adjustments apply to cephalexin?
When creatinine clearance is 30–59 mL/min, maximum daily dose should not exceed 1 g. For 15–29 mL/min use 250 mg every 8 or 12 hours; for 5–14 mL/min not on dialysis use 250 mg every 24 hours; for 1–4 mL/min use 250 mg every 48 or 60 hours per Table 1 in prescribing information.
What adverse effects matter most with cephalexin?
Serious hypersensitivity including anaphylaxis is highest stakes. Common effects include diarrhea, nausea, vomiting, and abdominal pain. Also monitor for C. difficile-associated diarrhea, drug-induced hemolytic anemia, seizures when dose is not reduced in renal impairment, and prolonged prothrombin time in at-risk patients.
Is cephalexin safe in pregnancy and breastfeeding?
Published epidemiologic data have not established increased risks of major birth defects or miscarriage with cephalosporin use including cephalexin, but studies have limitations. Cephalexin is present in human milk with relative infant dose under 1% of maternal weight-adjusted dose; consider maternal need and potential infant effects.
References
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U.S. National Library of Medicine. Cephalexin capsules — Full prescribing information. DailyMed (Teva Pharmaceuticals USA, Inc.; revised 12/2020).https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=3fed2817-fc0d-43cd-9f02-948fbc76672c
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National Institute for Health and Care Excellence. Cefalexin. BNF.https://bnf.nice.org.uk/drugs/cefalexin/
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Drugs and Lactation Database (LactMed). Cephalexin. Bethesda (MD): National Institute of Child Health and Human Development.https://www.ncbi.nlm.nih.gov/books/NBK482733/
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Centers for Disease Control and Prevention. Clostridioides difficile Infection — Information for clinicians.https://www.cdc.gov/cdiff/clinicians/diagnosis.html
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U.S. National Library of Medicine. Cephalexin. MedlinePlus.https://medlineplus.gov/druginfo/meds/a682733.html
Review and transparency
This medication guide is written and reviewed using NurseOnShift editorial and clinical review standards.
Educational use only. This content does not replace clinical judgment, prescriber orders, pharmacist guidance, product labeling, or institutional protocols.