Keflex. What diseases does it treat?

Use Keflex (cephalexin) for outpatient infections caused by streptococci and methicillin‑susceptible Staphylococcus aureus (MSSA). Indications with clear evidence of benefit include uncomplicated skin and soft tissue infections (cellulitis, impetigo), streptococcal pharyngitis, uncomplicated cystitis, and acute otitis media when the likely pathogens are susceptible. Start therapy based on clinical diagnosis; obtain cultures for purulent or severe infections and switch to targeted therapy if susceptibilities differ.

Dosing guidance: Adults: 250–500 mg orally every 6 hours (total 1–2 g/day) for most outpatient indications. Pediatrics: 25–50 mg/kg/day divided every 6–12 hours, not exceeding typical adult maxima. Duration by syndrome: streptococcal pharyngitis – 10 days; uncomplicated cystitis – 5–7 days; skin and soft tissue infections – commonly 7–10 days (shorten or extend based on clinical response). Adjust therapy sooner for rapid improvement or extend when resolution is slow, guided by clinical signs and culture results.

Activity profile: cephalexin reliably covers streptococci, MSSA and many Enterobacterales (Escherichia coli, Klebsiella, Proteus). It does not provide dependable coverage for MRSA, Pseudomonas aeruginosa, or enterococci; avoid for suspected infections with these organisms. Choose an alternative agent or obtain cultures when risk factors for resistant pathogens exist (recent hospitalization, prior MRSA, recent broad‑spectrum antibiotic exposure).

Safety and interactions: avoid cephalexin in patients with a history of immediate hypersensitivity to cephalosporins or severe penicillin allergy; use an allergy consultation if uncertain. Reduce dose or extend dosing interval for significant renal impairment (consider modification when creatinine clearance is low; consult renal dosing charts). Probenecid decreases renal excretion and increases cephalexin levels. Common adverse effects include gastrointestinal upset and diarrhea; report severe rash, angioedema, or signs of Clostridioides difficile infection and stop the drug if these occur.

Pregnancy and special considerations: cephalexin carries a favorable safety profile in pregnancy and is commonly used when indicated. For severe infections (bacteremia, osteomyelitis, endocarditis), prefer parenteral agents with proven tissue penetration and tailor therapy using culture data and infectious disease input.

Skin and soft tissue infections: causative organisms, dosing options, and typical treatment length

Use cephalexin when MSSA or streptococci are the suspected pathogens; typical adult dosing is 250–500 mg orally every 6 hours (total 1–2 g/day), with duration tailored to infection type and clinical response.

Common causative organisms

Nonpurulent cellulitis: most often Streptococcus pyogenes and related streptococci; MSSA can also cause cellulitis with broken skin. Purulent infections (abscesses, furuncles): frequently MSSA and MRSA; cephalexin covers MSSA but not MRSA, so choose empiric therapy based on local MRSA prevalence and risk factors. Impetigo: Strep pyogenes and MSSA. Animal and human bites: Pasteurella multocida (animal), Eikenella corrodens (human) and mixed oral flora–cephalexin provides poor coverage for these and is not recommended as monotherapy. Diabetic foot and severe soft-tissue infections: often polymicrobial including gram-negatives and anaerobes; cephalexin alone is usually insufficient.

Dosing options and typical treatment length

Adults – mild, nonpurulent cellulitis or impetigo: 250–500 mg PO every 6 hours (1–2 g/day) for 7 days for impetigo and 5 days for uncomplicated cellulitis; extend to 7–10 days if improvement is incomplete. More extensive or deeper MSSA cellulitis: 500 mg PO every 6 hours (2 g/day); reassess after 48–72 hours and continue 5–10 days total depending on response. Purulent SSTI with adequate incision and drainage and no systemic signs: antibiotics may be unnecessary; if treating for suspected MSSA use cephalexin 500 mg PO every 6 hours for 5–7 days. Purulent SSTI with systemic signs or multiple lesions: treat 5–10 days and select MRSA-active therapy if MRSA risk is present.

Children – typical dosing 25–50 mg/kg/day divided every 6–8 hours (maximum usual total 1–2 g/day); for more severe infections providers may use 75–100 mg/kg/day divided q6h. Impetigo oral courses commonly run 7 days. Adjust pediatric duration based on lesion count and clinical improvement.

Renal impairment – adjust dosing frequency for moderate–severe renal dysfunction; increase dosing interval for patients with reduced creatinine clearance (for example, extending to q12–24h depending on degree of impairment and dose). Avoid cephalexin monotherapy for bite wounds, suspected MRSA, or polymicrobial diabetic foot infections; choose agents with appropriate anaerobic and gram-negative coverage or combine therapies as indicated.

Uncomplicated urinary tract infections: indications for Keflex and standard oral regimens

Use cephalexin (Keflex) for acute uncomplicated cystitis when the pathogen is known to be susceptible or when first‑line oral agents (nitrofurantoin, trimethoprim–sulfamethoxazole, fosfomycin) are contraindicated, unavailable, or not tolerated.

• Typical indications
  • Acute uncomplicated lower UTI (cystitis) in non‑pregnant adults when culture/sensitivity supports cephalexin.
  • Pregnancy: commonly chosen because of safety profile when susceptibility is documented or empiric alternatives are unsuitable.
  • Patients with intolerance or allergy to first‑line agents (avoid in patients with immediate, severe beta‑lactam hypersensitivity).
• Usual causative organisms with likely activity
  • Escherichia coli (variable susceptibility by region).
  • Proteus mirabilis, Klebsiella spp., Staphylococcus saprophyticus (often susceptible).
  • Do not use empirically if local rates of resistance or prevalence of ESBL‑producing Enterobacterales are high.
• Oral dosing options and durations (adult outpatient)
  • Common regimen: cephalexin 500 mg orally twice daily for 5–7 days.
  • Alternative dosing: 250–500 mg orally every 6–8 hours for 5 days (use higher end for more severe symptoms).
  • Pregnancy: many clinicians use 500 mg every 8 hours for 7 days when indicated; tailor to culture results.
  • Men or suspected upper tract infection: choose agents with proven renal tissue penetration and extend duration (usually 7–14 days); cephalexin is not preferred for pyelonephritis without susceptibility confirmation.
• Practical management points
  • Obtain a urine culture before starting therapy when possible for recurrent, complicated, or atypical presentations; switch to targeted therapy based on susceptibilities.
  • Expect symptom improvement within 48–72 hours; if symptoms persist or worsen, reassess and obtain or repeat culture.
  • Adjust dose or dosing interval in significant renal impairment (reduce frequency if creatinine clearance is reduced).
  • Avoid cephalexin for infections caused by known ESBL producers or organisms resistant to first‑generation cephalosporins.
  • Adverse effects: common–gastrointestinal upset; less common–hypersensitivity reactions and risk of Clostridioides difficile infection.

Upper and lower respiratory infections: when Keflex is appropriate and alternatives to consider

Use cephalexin (Keflex) for respiratory infections strongly suspected or confirmed to be caused by Streptococcus spp. or methicillin-susceptible Staphylococcus aureus (MSSA); avoid it when Haemophilus influenzae, Moraxella catarrhalis, or atypical pathogens are likely.

Pharyngitis: prescribe cephalexin for confirmed group A Streptococcus in patients without immediate-type penicillin allergy. Typical adult dose: 500 mg twice daily or 250–500 mg every 6 hours for 10 days. For children: 25–50 mg/kg/day divided every 6–12 hours (max 4 g/day) for 10 days. First-line alternative: phenoxymethylpenicillin or amoxicillin; use azithromycin only with documented beta-lactam allergy and consider local macrolide resistance.

Acute otitis media: avoid routine use of cephalexin because H. influenzae and M. catarrhalis commonly produce beta-lactamases. Consider cephalexin only when culture shows penicillin-susceptible streptococci or MSSA and after failure of high-dose amoxicillin. Typical adult dosing mirrors pharyngitis; pediatric dosing as above for 5–10 days depending on age and severity. Preferred alternatives: high-dose amoxicillin or amoxicillin-clavulanate; azithromycin or doxycycline for appropriate allergies.

Acute bacterial rhinosinusitis: reserve cephalexin for culture-proven streptococcal sinusitis or post-surgical wound infections; avoid empiric use due to limited activity against beta-lactamase–producing H. influenzae and M. catarrhalis. Usual adult duration: 5–7 days for uncomplicated cases with clinical improvement; extend to 10 days if response is slow. First-line empiric therapy: amoxicillin-clavulanate; doxycycline or a respiratory macrolide are alternatives when indicated by allergy patterns and local resistance.

Acute bronchitis and COPD exacerbations: do not use cephalexin for uncomplicated acute bronchitis because viral causes predominate and atypical organisms may be involved. For bacterial exacerbations of chronic bronchitis, choose agents with activity against H. influenzae and M. catarrhalis (amoxicillin-clavulanate, doxycycline, or macrolides). Use cephalexin only when sputum culture isolates show susceptible streptococci or MSSA.

Community-acquired pneumonia (outpatient): avoid cephalexin as monotherapy. It lacks reliable activity against common respiratory pathogens such as S. pneumoniae with intermediate resistance and atypicals (Mycoplasma, Chlamydophila). Preferred outpatient regimens include amoxicillin ± macrolide or doxycycline; respiratory fluoroquinolones in selected cases.

Allergy and safety: consider cephalosporins for patients with non–immediate penicillin rashes. Avoid cephalexin in patients with a history of immediate hypersensitivity (anaphylaxis) to penicillins unless allergy evaluation supports safe use. Adjust dose for renal impairment; maximum usual adult daily dose 4 g.

Infection	When Keflex appropriate	Typical adult dosing & duration	Preferred empiric alternatives
Streptococcal pharyngitis	Confirmed GAS or suspected streptococcal cause, no severe penicillin allergy	500 mg twice daily or 250–500 mg q6h × 10 days	Penicillin V or amoxicillin
Acute otitis media	Post-culture streptococcal or MSSA isolates; not first-line	250–500 mg q6h; 5–10 days by age and severity	High-dose amoxicillin; amoxicillin-clavulanate
Acute bacterial sinusitis	Culture-proven streptococcal sinusitis or surgical site isolates	250–500 mg q6–12h; 5–7 days if improved, up to 10 days if needed	Amoxicillin-clavulanate; doxycycline (allergy)
Bronchitis / COPD exacerbation	Bacterial exacerbation with sputum culture showing susceptible streptococci or MSSA	As above; typically 5–7 days	Amoxicillin-clavulanate; doxycycline; macrolide
Community-acquired pneumonia (outpatient)	Not recommended as monotherapy	Not recommended	Amoxicillin ± macrolide or doxycycline; respiratory fluoroquinolone if indicated

Bone and joint infections: oral therapy role, duration considerations, and monitoring parameters

Use oral cephalexin only when bone or joint infection is proven or highly likely to be caused by susceptible organisms (MSSA or beta‑hemolytic streptococci), the patient shows clinical improvement after any necessary initial IV therapy, and reliable oral absorption and close outpatient follow‑up are available.

Obtain pathogen identification before committing to an oral regimen: synovial fluid or bone culture (preferably bone biopsy if safe) should guide therapy. Avoid oral cephalexin for MRSA, Pseudomonas, enteric Gram‑negatives, anaerobic mixed infections, or culture‑negative cases where broad coverage is needed.

Typical oral dosing: 500 mg every 6 hours (2 g/day) is a common regimen; some clinicians increase to 1 g every 6 hours (up to 4 g/day) for severe MSSA bone infections with documented susceptibility. Adjust dose for renal impairment (CrCl‑based reductions). Use lower doses for mild infections only after culture confirmation of susceptibility.

Duration by scenario: acute hematogenous osteomyelitis–generally 4–6 weeks total therapy; chronic or osteomyelitis with retained infected bone–commonly ≥6 weeks and sometimes longer guided by response; diabetic foot osteomyelitis–2–3 weeks after complete surgical resection of infected bone, 6 weeks or more if conservative management or residual infected bone; native septic arthritis–typically 2–4 weeks total, with at least an initial period of parenteral therapy for most patients; prosthetic joint infection–rarely managed with oral monotherapy alone; if implant retention is chosen, combine surgical strategy with agents active against biofilm (rifampin‑based combinations for staphylococci) and extend therapy per orthopedic/ID guidance.

Criteria for switching from IV to oral: clinically improving pain and function, afebrile for 48–72 hours, down‑trending inflammatory markers (CRP preferred), documented susceptible organism, functioning gastrointestinal tract, and reliable adherence. If these are absent, continue or reescalate parenteral treatment.

Monitoring during oral therapy: baseline and periodic CRP (expect a measurable decline within 1–2 weeks; failure to fall by 2 weeks should prompt reassessment), ESR for adjunctive trend if used, weekly CBC for prolonged courses, baseline and periodic creatinine for renal dosing, and baseline LFTs with interval checks for long courses or concomitant hepatotoxic drugs. Perform clinical assessments at 1–2 week intervals initially (pain, wound/drainage, limb function, temperature). If signs persist or worsen, repeat cultures and obtain MRI or targeted imaging to reassess extent and guide surgical decisions.

Watch for adverse effects: new or progressive rash (stop and evaluate for allergy), diarrhea and Clostridioides difficile risk, and signs of treatment failure (persistent fever, drainage, progressive bone destruction on imaging). If biofilm infection or prosthetic involvement is suspected, avoid prolonged oral monotherapy without specialist input and targeted combination therapy.

Pediatric indications: age‑based dosing, common diagnoses treated, and administration tips

Prescribe cephalexin 25–50 mg/kg/day divided every 8–12 hours for most outpatient pediatric infections; adjust frequency and dose for neonates, renal impairment, and specific clinical scenarios and limit total daily dose to adult maximums when treating adolescents.

Age-based dosing and practical examples

Standard outpatient range: 25–50 mg/kg/day in 2–4 divided doses (every 8–12 hours). Use the lower end for mild infections and the higher end for more symptomatic disease or poor tissue penetration. For infants under 1 month, reduce dosing frequency because renal clearance is immature; consult neonatal dosing charts or a pediatric pharmacist for exact intervals. For children with significant renal impairment extend the dosing interval; check renal dosing references for creatinine clearance thresholds.

Examples (using 25–50 mg/kg/day):

- 10 kg child: 250–500 mg/day → 125 mg every 12 hours or 62.5–125 mg every 8 hours.

- 20 kg child: 500–1,000 mg/day → 250 mg every 12 hours or 166–333 mg every 8 hours (round to available formulations).

- 30 kg adolescent: 750–1,500 mg/day → 375 mg every 12 hours or 187–375 mg every 8 hours (use available capsule sizes or suspension).

Common pediatric formulations: oral suspension 125 mg/5 mL (25 mg/mL) and 250 mg/5 mL (50 mg/mL), and capsules (250 mg, 500 mg). Convert weight-based dose to mL using the suspension concentration when a syringe is used for dosing.

Administration, safety, and adherence tips

Measure liquid doses with an oral syringe; shake the bottle thoroughly before each dose. Give doses with a small amount of food or milk to reduce gastrointestinal upset if the child refuses medication on an empty stomach. Refrigerate reconstituted suspension and discard after 14 days unless the manufacturer specifies otherwise.

Address palatability and adherence: use an oral syringe into the cheek pocket for toddlers, offer a preferred cold drink immediately after, and schedule doses around sleep and meals to improve consistency. For school dosing, provide written instructions and an appropriately labeled syringe or premeasured cups for caregivers and school staff.

Allergy and adverse reaction guidance: do not give cephalexin to children with a history of immediate-type (anaphylactic) penicillin allergy; for nonsevere delayed rashes discuss risks with the prescriber. Stop the drug and seek immediate care for hives, facial swelling, difficulty breathing, or signs of severe cutaneous reaction. Expect mild gastrointestinal upset and transient rash; persistent bloody diarrhea warrants medical evaluation for C. difficile.

Missed dose rule: give the missed dose as soon as remembered unless the next dose is close, then skip the missed dose and resume schedule; do not double doses. For prolonged courses (>10–14 days) or in children with renal disease, arrange repeat evaluation and consider basic laboratory monitoring (renal function, complete blood count) as clinically indicated.