KEFLEX
Clinical safety rating: caution
Comprehensive clinical and safety monograph for KEFLEX (KEFLEX).
Cephalexin is a first-generation cephalosporin antibiotic that inhibits bacterial cell wall synthesis by binding to penicillin-binding proteins (PBPs), leading to cell lysis and death.
| Metabolism | Cephalexin is not extensively metabolized; it is primarily excreted unchanged in the urine via renal tubular secretion and glomerular filtration. Minor metabolism may occur via hydrolysis. |
| Excretion | Primarily renal (90% or more unchanged via glomerular filtration and tubular secretion); small amounts biliary/fecal (<5%). |
| Half-life | 0.5–1.2 hours in patients with normal renal function (CrCl >50 mL/min); prolonged to >20 hours in ESRD. |
| Protein binding | 6–10% bound to plasma proteins (primarily albumin). |
| Volume of Distribution | 0.2–0.4 L/kg; indicates distribution primarily into extracellular fluid. |
| Bioavailability | Oral: ~90–95% (well absorbed); IM: ~100%. |
| Onset of Action | Oral: ~30–60 minutes; IV: immediate. |
| Duration of Action | 6–8 hours for susceptible organisms; dose adjustment needed for renal impairment. |
| Molecular Weight | 347.39 |
| Action Class | Cephalosporins: 1st generation |
| Brand Substitutes | Seafal 500mg Capsule, Q Cef 500mg Capsule, Equitrol Forte 500mg Capsule, Yescef 500mg Capsule, Nexporin 500 Capsule, Cephalkem 250mg Capsule, Equitrol 250mg Capsule, Nexporin 250 Capsule, Apkef 250mg Capsule, Cefrik 250mg Capsule |
250-500 mg orally every 6 hours; maximum 4 g/day.
| Dosage form | FOR SUSPENSION |
| Renal impairment | CrCl 30-49 mL/min: 500 mg every 8-12 hours; CrCl 10-29 mL/min: 250-500 mg every 12-24 hours; CrCl <10 mL/min: 250 mg every 24 hours. |
| Liver impairment | No dosage adjustment required for hepatic impairment; no Child-Pugh based modifications established. |
| Pediatric use | 25-50 mg/kg/day orally divided every 6-8 hours; for otitis media: 75 mg/kg/day divided every 12 hours. |
| Geriatric use | Start at lower end of dosing range; monitor renal function; adjust based on creatinine clearance. |
| 1st trimester | Generally considered safe; no evidence of teratogenicity in human studies. Use if clearly needed. |
| 2nd trimester | Safe; no known fetal risks. Commonly used for UTIs and other infections. |
| 3rd trimester | Safe; no reported adverse fetal effects. Cephalosporins are commonly used in pregnancy. |
Clinical note
Comprehensive clinical and safety monograph for KEFLEX (KEFLEX).
| Placental transfer | Cephalexin crosses the placenta, achieving fetal serum levels approximately 10-30% of maternal levels. The degree of transfer is moderate but has not been associated with fetal harm. |
| Breastfeeding | Cephalexin is excreted into breast milk in low amounts (less than 1% of maternal dose). No adverse effects reported in infants. Considered compatible with breastfeeding. Monitor for potential diarrhea or allergic reaction in the infant. |
■ FDA Black Box Warning
No FDA black box warning.
| Serious Effects |
Hypersensitivity to cephalexin or other cephalosporinsHistory of anaphylactic reaction to penicillins (possible cross-sensitivity)
| Precautions | Hypersensitivity reactions, including anaphylaxis, Clostridioides difficile-associated diarrhea, Renal impairment: dose adjustment required, Seizure potential with high doses or renal insufficiency, Prolonged use may result in superinfection, False-positive urine glucose tests with non-enzymatic methods, Use with caution in patients with history of penicillin allergy (cross-sensitivity) |
| Food/Dietary | Take with food or milk to reduce GI upset. No specific food-drug interactions; avoid alcohol. |
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| Lactation Rating | L1 (Safe) |
| Teratogenic Risk | FDA Pregnancy Category B. Animal studies show no fetal harm. No adequate human studies in first trimester; cephalosporins generally considered low risk. No known teratogenicity in any trimester. |
| Fetal Monitoring | Monitor maternal renal function and for signs of hypersensitivity or Clostridioides difficile-associated diarrhea. No fetal-specific monitoring required unless maternal infection warrants. |
| Fertility Effects | No adverse effects on fertility reported in animal or human studies. |
| Clinical Pearls |
| Keflex (cephalexin) is a first-generation cephalosporin effective against Gram-positive cocci (e.g., Streptococcus pyogenes, methicillin-susceptible Staphylococcus aureus). It is renally eliminated; dose adjustment required for CrCl <30 mL/min. Poor CNS penetration, so not for meningitis. Can cause false-positive urine glucose test (Clinitest) but not glucose-specific tests. May reduce oral contraceptive efficacy; consider backup contraception. |
| Patient Advice | Take exactly as prescribed; complete full course even if feeling better. · Take with food or milk if gastrointestinal upset occurs. · Avoid alcohol during treatment and for 72 hours after last dose. · Report any signs of allergy (rash, hives, difficulty breathing) or severe diarrhea. · May reduce effectiveness of birth control pills; use additional contraceptive method. · Store at room temperature; keep bottle tightly closed. |