Cephalexin. What diseases does it treat?

Use cephalexin for outpatient infections caused by methicillin-susceptible Staphylococcus aureus and Streptococcus species, and for many uncomplicated gram-negative infections when susceptibility is confirmed. For adults a common regimen is 250–500 mg every 6 hours (total 1–2 g/day); for more severe but still oral-treatable skin infections consider 500 mg every 6–8 hours (up to 4 g/day). Start therapy after culture/sensitivity when possible and switch from IV to oral cephalexin only if clinical response and organism susceptibility permit.

Cephalexin reliably treats: uncomplicated skin and soft-tissue infections (impetigo, cellulitis), streptococcal pharyngitis (standard 10-day course to eradicate carriage), selected cases of otitis media and sinusitis when organisms are susceptible, and uncomplicated urinary tract infections caused by susceptible E. coli, Klebsiella or Proteus. Typical durations: skin infections 5–14 days depending on severity, streptococcal pharyngitis 10 days, uncomplicated cystitis 5–7 days. For children use 25–50 mg/kg/day divided every 6–12 hours (up to 75–100 mg/kg/day for more severe infections); round doses to available formulations and verify weight-based calculations.

Watch for allergy, renal function and resistance patterns. Do not give cephalexin to patients with a history of anaphylaxis to beta-lactams without allergy evaluation; cross-reactivity with penicillins is low but possible. Adjust dosing in renal impairment by extending the dosing interval or reducing dose when creatinine clearance is reduced. Expect common adverse events such as gastrointestinal upset and transient rash; discontinue and evaluate immediately for signs of severe hypersensitivity or C. difficile–associated diarrhea. Probenecid raises cephalexin levels; check interactions if co-prescribed.

Practical tips: obtain cultures for nonresponse or severe presentations, avoid cephalexin for suspected MRSA or beta-lactamase–producing respiratory pathogens, and consider oral cefalexin only when local antibiogram shows susceptibility. Counsel patients to complete prescribed courses, report new rashes or severe diarrhea, and seek urgent care for breathing difficulty or facial swelling.

Skin and Soft Tissue Infections: Typical Pathogens, Dosing Strategies, and Duration

Use cephalexin for suspected MSSA or streptococcal skin infections; avoid it when MRSA, anaerobic, or gram‑negative coverage is required.

Pathogens and clinical indications

Common pathogens by presentation: cellulitis – Streptococcus pyogenes and methicillin‑susceptible Staphylococcus aureus (MSSA); impetigo – S. pyogenes and S. aureus; uncomplicated abscesses – S. aureus (MRSA frequent, so tailor therapy after drainage and culture); animal or human bite wounds – Pasteurella, Eikenella, mixed anaerobes (cephalexin alone does not cover these reliably); diabetic foot and chronic ulcer infections – mixed flora including gram‑negatives and anaerobes (broader agents preferred). Choose cephalexin when clinical features and local epidemiology support MSSA/streptococcal etiology and there are no risk factors for MRSA, gram‑negative, or anaerobic infection.

Dosing strategies, renal adjustment, monitoring

Adult dosing: 500 mg orally every 6 hours (common) or 250–500 mg every 6–12 hours based on severity; maximum usual daily dose 4 g. Pediatric dosing: 25–50 mg/kg/day divided every 6–12 hours (typical range 12.5–17 mg/kg per dose q8h or q6h in severe cases), not exceeding adult maximum. Renal impairment: for creatinine clearance 10–29 mL/min consider 250–500 mg every 12 hours; for CrCl <10 mL/min consider 250–500 mg every 24 hours. Obtain wound culture for purulent infections or treatment failures and switch therapy based on culture and susceptibility results. In patients with severe immediate beta‑lactam allergy, use non‑beta‑lactam alternatives guided by likely pathogen.

Clinical scenario	Typical pathogens	When cephalexin fits	Suggested adult dose & usual duration
Nonpurulent cellulitis	Streptococci, MSSA	First‑line oral option if MRSA risk low	500 mg PO q6–8h; 5 days if clear improvement by day 3–4, extend to 7–10 days if slow response
Impetigo (nonbullous)	S. pyogenes, MSSA	Good oral option for widespread lesions or systemic symptoms	250–500 mg PO q6–12h; 7 days
Purulent abscess after I&D	S. aureus (including MRSA risk)	Use if culture confirms MSSA and patient has risk factors for systemic spread	500 mg PO q6–8h; 5–7 days for limited disease, extend if deep/complicated
Bite wounds	Pasteurella, Eikenella, anaerobes	Not recommended as monotherapy	Prefer amoxicillin‑clavulanate; tailor duration 5–7 days for mild, longer if deep/osteomyelitis
Diabetic foot / chronic ulcers	Polymicrobial incl. gram‑negatives, anaerobes	Not suitable as single agent for moderate–severe infections	Use broader coverage based on culture and severity; specialist input for bone involvement

Escalate therapy or obtain specialty consultation for systemic toxicity, rapid progression, failure to improve within 48–72 hours, suspected deep extension (lymphangitis, abscess not drained, necrotizing infection), or diabetic foot/osteomyelitis. Adjust dose for renal function and switch to pathogen‑directed oral therapy when cultures permit.

Uncomplicated Urinary Tract Infections: Indications for Cephalexin and Recommended Regimens

Reserve cephalexin for acute uncomplicated cystitis when local or patient-specific Enterobacterales susceptibility to first‑generation cephalosporins is documented or when first‑line oral agents (nitrofurantoin, TMP‑SMX, fosfomycin) are contraindicated or unavailable; typical adult regimen: 500 mg PO twice daily for 5–7 days.

• When to choose cephalexin
  • Urine culture shows an organism susceptible to cephalexin (or cefazolin) and clinical presentation is limited to lower urinary tract symptoms.
  • Patient has contraindication to nitrofurantoin (e.g., CrCl <30 mL/min), TMP‑SMX (history of severe reaction or high local resistance), or fosfomycin is unavailable.
  • Pregnancy: acceptable oral option for asymptomatic bacteriuria or cystitis when culture supports susceptibility.
• When to avoid cephalexin
  • Suspected or proven pyelonephritis (prefer agents with reliable renal parenchyma penetration and higher gram‑negative coverage; consider oral fluoroquinolones only if susceptibility known and appropriate).
  • Complicated UTI (indwelling catheter, urinary tract abnormality, recent urologic procedure) unless guided by culture and specialist input.
  • History of immediate IgE‑mediated anaphylaxis to cephalosporins or severe beta‑lactam allergy.
• Recommended adult dosing (nonpregnant)
  1. Standard: 500 mg PO twice daily for 5–7 days for uncomplicated cystitis.
  2. Alternative split dosing: 250 mg PO every 6 hours for 5–7 days if twice‑daily dosing not tolerated.
  3. Shorter courses (3 days) lack robust evidence for cephalexin; prefer 5 days when using this agent.
• Pediatric dosing (acute cystitis)
  • 25–50 mg/kg/day divided every 6–8 hours (max total daily dose ~4 g) for 5–7 days; adjust per weight and clinical response.
• Pregnancy
  • Treat asymptomatic bacteriuria or symptomatic cystitis with 500 mg PO every 8–12 hours for 7 days when culture demonstrates susceptibility.
• Renal function adjustments
  • Cephalexin is renally excreted; reduce dose or extend dosing interval for reduced CrCl.
  • Practical approach: CrCl 30–50 mL/min use standard dose but consider twice‑daily dosing; CrCl <30 mL/min extend interval (e.g., 250–500 mg every 12–24 hours) and consult local renal dosing guidance.
• Microbiology and stewardship points
  • E. coli resistance to first‑generation cephalosporins varies by region; choose cephalexin only if susceptibility rates are acceptable (local resistance <20%) or individual isolate is susceptible.
  • Obtain urine culture for recurrent infection, recent antibiotic exposure, male patients, pregnancy, or treatment failure within 48–72 hours.
  • Switch to narrower‑spectrum targeted therapy when culture and sensitivities return; stop therapy if culture negative and symptoms resolve.
• Monitoring and safety
  • Assess symptom improvement within 48–72 hours; if no improvement, obtain culture and consider alternative agents or escalation to parenteral therapy for suspected upper tract involvement.
  • Watch for signs of Clostridioides difficile–associated diarrhea and discontinue if severe diarrhea develops.
  • In patients with non‑anaphylactic penicillin allergy, cephalexin is often tolerated; avoid in documented immediate severe beta‑lactam hypersensitivity.

Respiratory Tract Infections: When Cephalexin Is Appropriate and Its Microbial Coverage

Use cephalexin for confirmed group A streptococcal (GAS) pharyngitis and for upper respiratory infections with documented susceptible Gram-positive pathogens; avoid empiric use for suspected community-acquired pneumonia, atypical pathogens, or infections likely caused by beta-lactamase–producing Gram-negative organisms.

• Microbial coverage – reliable:
  • Streptococcus pyogenes (GAS) – high susceptibility; cephalexin achieves bactericidal concentrations in pharyngeal tissue.
  • Methicillin-susceptible Staphylococcus aureus (MSSA) – predictable activity for superficial respiratory-associated staphylococcal isolates.
  • Group C and G streptococci – generally susceptible.
• Microbial coverage – variable or limited:
  • Streptococcus pneumoniae – susceptibility varies by region due to altered penicillin-binding proteins; check local resistance data before use for lower respiratory infections.
  • Haemophilus influenzae and Moraxella catarrhalis – many strains produce beta-lactamase and will be resistant; cephalexin has limited utility against beta-lactamase–positive isolates.
• No clinically useful activity:
  • Atypical respiratory pathogens (Mycoplasma pneumoniae, Chlamydophila pneumoniae, Legionella) – cephalexin lacks activity and should not be used when these are suspected.

• When to choose cephalexin:
  • Confirmed GAS pharyngitis by rapid antigen test or throat culture.
  • Upper respiratory infections with culture-proven MSSA or streptococci and no signs of lower respiratory tract involvement.
  • Patients with non–IgE-mediated penicillin reactions (rash) where a first-generation cephalosporin is acceptable; avoid in immediate IgE-mediated penicillin allergy.
• When not to choose cephalexin:
  • Empiric treatment of community-acquired pneumonia, especially if atypical pathogens are likely or local pneumococcal resistance is high.
  • Acute otitis media or acute bacterial sinusitis when beta-lactamase–producing organisms are suspected–amoxicillin-clavulanate is preferable.
  • Severe lower respiratory tract infections requiring broader coverage or agents with good intracellular penetration.

• Dosing guidance (typical ranges):
  • Adults: 250–500 mg every 6 hours (total 1–2 g/day). For more severe infections clinicians may use up to 1 g every 6 hours (max 4 g/day).
  • Children: 25–50 mg/kg/day divided every 6–12 hours (maximum ~4 g/day).
  • Duration: GAS pharyngitis – 10 days. For other documented susceptible upper respiratory infections use 7–10 days guided by clinical response and culture results; extend therapy if deep-seated infection or slow response.
• Diagnostic and stewardship recommendations:
  • Confirm GAS with rapid antigen testing or throat culture before prescribing for pharyngitis unless high clinical likelihood and testing not available.
  • Obtain culture and susceptibility when symptoms fail to improve or when lower respiratory infection is suspected; tailor therapy to results.
  • Consult local antibiograms for pneumococcal susceptibility before considering cephalexin for lower respiratory isolates.
• Safety and precautions:
  • Avoid in patients with a history of immediate IgE-mediated penicillin allergy; use an alternative class (macrolide, doxycycline) guided by age and pathogen.
  • Common adverse effects: gastrointestinal upset, rash. Monitor for Clostridioides difficile–associated diarrhea with any antibiotic exposure.

Bone and Joint Infections: Role of Cephalexin, Oral Step-Down Criteria, and Treatment Length

Use cephalexin as an oral step-down for osteomyelitis or septic arthritis only when the isolate is susceptible (MSSA or penicillin‑susceptible streptococci), source control has been achieved, and the patient is clinically improving and tolerating oral intake.

Oral step-down criteria

Transition from IV to oral cephalexin when all the following apply: pathogen confirmed and susceptible by culture and MIC reporting; fever has resolved for 48–72 hours; inflammatory markers (CRP trending down; ESR may lag) show clear improvement; surgical drainage, debridement or device removal/management has been performed when indicated; reliable gastrointestinal absorption and no vomiting; patient adherent with outpatient follow-up and able to take frequent dosing. Avoid oral transition if MRSA, Pseudomonas or mixed Gram‑negative/anaerobic infection predominates, or if hardware is retained without rifampin-containing combination therapy for staphylococcal biofilm.

Treatment length, dosing and monitoring

Typical durations: acute hematogenous or contiguous osteomyelitis: 4–6 weeks total antimicrobial therapy (count IV days toward the total); vertebral osteomyelitis: minimum 6 weeks; septic arthritis after adequate drainage: 2–4 weeks (shorter courses acceptable for rapid responders); prosthetic joint infection (PJI): if implant is retained and organism is MSSA, use a rifampin-containing regimen and plan prolonged therapy (frequently ≥3 months); if prosthesis removed, aim for 4–6 weeks after resection. For oral cephalexin dosing, use 500 mg every 6 hours (2 g/day) for standard step-down; consider 1 g every 8 hours (3 g/day) for deeper or slow-responding infections if tolerated and isolate has low MIC. Reduce dose or extend interval in substantial renal impairment and check renal function periodically.

Monitor CRP weekly until stable, reassess wound/drainage sites and signs of relapse at 2–4 week outpatient visits, and review drug interactions before adding rifampin (rifampin 300 mg twice daily is commonly used in combination when indicated). Stop oral therapy and re-evaluate if clinical deterioration, rising CRP, new fever, or inability to tolerate oral dosing occurs.

Streptococcal Pharyngitis and Otitis Media: Comparative Use, Pediatric Dosing, and When to Choose Alternatives

Use cephalexin for confirmed streptococcal pharyngitis at 25–50 mg/kg/day divided every 6–12 hours for 10 days (maximum ~4 g/day); avoid routine use for acute otitis media (AOM) because Haemophilus influenzae and Moraxella catarrhalis commonly produce beta-lactamases and are poorly covered by cephalexin.

Comparative use and microbial coverage

Cephalexin reliably eradicates Group A Streptococcus (GAS) and treats streptococcal pharyngitis with similar clinical cure rates to penicillin derivatives when susceptibility is present. For AOM the dominant pathogens are Streptococcus pneumoniae, H. influenzae and M. catarrhalis. Cephalexin covers S. pneumoniae strains that are penicillin-susceptible and MSSA, but it lacks consistent activity against beta-lactamase–producing H. influenzae and M. catarrhalis; that gap explains higher failure rates when cephalexin is used empirically for uncomplicated AOM. Choose cephalexin for AOM only when culture/susceptibility data identify a susceptible pathogen or when a patient cannot receive first-line agents for non-allergic reasons and the clinician accepts the coverage limitations.

Pediatric dosing, durations, and when to select alternatives

Streptococcal pharyngitis: cephalexin 25–50 mg/kg/day divided q6–12h for 10 days (max ~4 g/day). Use a full 10-day course to ensure eradication of GAS and reduce carrier state. If the child has a non‑anaphylactic penicillin allergy, cephalexin is an appropriate alternative at the same dosing. For immediate-type (IgE‑mediated) penicillin allergy, use azithromycin or clindamycin: azithromycin 12 mg/kg once on day 1 (max 500 mg), then 6 mg/kg once daily on days 2–5; clindamycin 20 mg/kg/day divided q8h (range 10–30 mg/kg/day) for 10 days, mindful of local susceptibility and C. difficile risk.

Acute otitis media: prefer high‑dose amoxicillin (80–90 mg/kg/day divided BID) as first-line for most children. If beta-lactamase–producing pathogens, recent amoxicillin exposure, conjunctivitis with AOM, severe disease, or treatment failure are present, select amoxicillin–clavulanate or an oral cephalosporin with improved H. influenzae coverage (cefuroxime axetil, cefpodoxime, cefdinir) rather than cephalexin. If cephalexin is chosen despite limitations, use 25–50 mg/kg/day divided TID and match duration to age: <2 years → 10 days; 2–5 years → 7 days; ≥6 years → 5 days. For children with immediate penicillin allergy, use a macrolide or clindamycin for AOM when indicated, but check local macrolide resistance rates before relying on azithromycin.

Choose alternatives when: recent beta-lactam exposure (within 30 days), concurrent conjunctivitis (suggesting H. influenzae), tympanostomy tube otorrhea, severe systemic signs, or local resistance makes macrolides unreliable. Adjust selection based on culture results, clinical severity, and allergy type rather than empirical substitution of cephalexin for standard AOM regimens.