Cephalexin

Introduction Overview

Generic Name

cephalexin

Brand Name

Keflex

Dosage Form

Oral capsule, oral tablet, oral suspension

Drug Class

First generation cephalosporins

Pharmacokinetics

Absorption:
Cephalexin is well-absorbed after oral administration, with bioavailability reported to range between 90 and 95%. Peak plasma concentrations are typically reached within 1 hour following ingestion. Absorption is not significantly affected by food, although taking it with meals may reduce gastrointestinal discomfort. The drug exhibits linear pharmacokinetics across the commonly prescribed dosing range. Factors such as gastrointestinal motility, presence of disease states like malabsorption syndrome, or concomitant medications may alter absorption to some degree, but overall the oral bioavailability remains high, providing predictable plasma levels.

Distribution:
Once absorbed, cephalexin is widely distributed into body tissues and fluids. It has a relatively low protein-binding rate, approximately 10%, allowing a significant free drug concentration to exert antimicrobial effects. The volume of distribution ranges from 0.2 to 0.3 L/kg. Cephalexin penetrates most tissues including skin, soft tissues, respiratory tract, and bone, making it effective for infections in these locations. It crosses the placenta but is not significantly distributed into cerebrospinal fluid. Due to low lipid solubility and high polarity, penetration into the central nervous system is limited. The drug also achieves therapeutic concentrations in urine, which supports its use in urinary tract infections.

Metabolism:
Cephalexin undergoes minimal metabolism, with more than 80% of the administered dose excreted unchanged in the urine. The limited biotransformation reduces potential for drug interactions that arise from hepatic enzyme induction or inhibition. Minor hepatic metabolism may occur but does not contribute significantly to elimination. The lack of metabolic transformation also means cephalexin maintains its antimicrobial activity without formation of toxic metabolites.

Elimination:
The primary route of elimination for cephalexin is renal excretion via glomerular filtration and tubular secretion. Approximately 80-90% of the drug is recovered unchanged in urine within 6 to 8 hours after administration. The elimination half-life in adults with normal renal function is approximately 0.5 to 1.2 hours. In patients with renal impairment, clearance is reduced and half-life prolonged, necessitating dosage adjustments. The efficient renal elimination also makes cephalexin suitable for treating urinary tract infections as it attains high urinary concentrations. The drug has minimal enterohepatic recycling or biliary excretion.

Dosage Forms

Available Dosage Forms:
Keflex is available in multiple oral formulations to suit varied patient needs. These include:
- Oral capsules: 250 mg, 500 mg, and 750 mg strengths.
- Oral tablets: 250 mg and 500 mg strengths.
- Oral suspension (liquid): Usually supplied as 125 mg/5 mL and 250 mg/5 mL, allowing precise dosing especially in pediatric patients.

Dosages for Adults and Adolescents:
The usual adult dosage varies based on the infection's severity and site. Common dosing regimens range from 250 mg every 6 hours to 500 mg every 12 hours. In more severe infections or bone infections, doses up to 1 gram every 6 hours may be prescribed. For skin and soft tissue infections, doses typically range from 250 to 500 mg every 6 hours for 7 to 14 days. For urinary tract infections, 250 mg every 6 hours for 7 days is standard. Therapy duration and dose may be modified depending on clinical response.

Specific Patient Populations:
In pediatric patients older than 1 year, dosing is based on body weight, typically 25 to 50 mg/kg/day divided into doses every 6 to 8 hours, not exceeding adult maximum doses. For patients with renal impairment, dose adjustment is recommended to prevent accumulation due to reduced clearance; this is vital in moderating the risk of toxicity. Safety and efficacy in infants less than one year of age have not been well established. No dosage adjustments are generally required for hepatic impairment because of minimal metabolism.

Mechanism Of Action

Cephalexin is a beta-lactam antibiotic belonging to the first generation cephalosporin class, exerting bactericidal effects by inhibiting bacterial cell wall synthesis. At the molecular level, cephalexin binds to penicillin-binding proteins (PBPs) located on the bacterial cell membrane. These PBPs are enzymes responsible for cross-linking peptidoglycan strands, which confer mechanical strength to the bacterial cell wall. By irreversibly binding to PBPs, cephalexin inhibits the transpeptidation reaction essential for cell wall synthesis, ultimately destabilizing the cell wall structure.

This disruption leads to impaired formation of the peptidoglycan layer, weakening the bacterial cell wall and making it susceptible to osmotic pressure changes. Resultant bacterial lysis and cell death ensue as the bacteria fail to maintain structural integrity. Cephalexin is particularly effective against gram-positive organisms due to its affinity for PBPs in these bacteria and provides moderate activity against some gram-negative species.

At the cellular level, this mechanism inhibits bacterial replication and proliferation, thereby reducing infection burden. The time-dependent killing property of cephalexin requires maintaining plasma concentrations above the minimum inhibitory concentration (MIC) for the bacteria for effective antibacterial activity. Clinically, this necessitates appropriately timed dosing regimens to sustain drug levels.

Comparatively, cephalexin shares a similar mechanism with penicillins but tends to have a broader spectrum of activity against certain gram-positive cocci and some gram-negative rods. However, it remains susceptible to degradation by beta-lactamase enzymes produced by resistant bacteria, limiting activity against beta-lactamase-producing organisms. Research has also explored modifications to cephalexin to enhance beta-lactamase stability, but classical cephalexin remains widely used for susceptible infections due to its efficacy and safety profile.

Pharmacological Actions

Bactericidal action via inhibition of bacterial cell wall synthesis.
Effective against gram-positive cocci including Streptococcus and Staphylococcus species.
Moderate activity against some gram-negative bacteria like Escherichia coli and Proteus mirabilis.
Time-dependent antimicrobial activity requiring plasma concentrations above MIC for efficacy.
Renally excreted with minimal metabolism, decreasing drug-drug interaction potential.
Rapid oral absorption and wide distribution into body tissues.
Low protein-binding enhancing free active drug concentration.
Resistance mainly due to beta-lactamase production by bacteria.
Used in prophylaxis and treatment of bacterial infections.
Minimal impact on intestinal microflora when taken appropriately.
Does not have activity against MRSA or Pseudomonas aeruginosa.
Does not accumulate significantly in hepatic impairment.
Concentration achieved in urine sufficient for urinary tract infection treatment.
Synergistic effects when combined with other antibiotics for mixed infections.
Minimal effect on cytochrome P450 enzymes.

Therapeutic Or Clinical Uses

Treatment of upper respiratory tract infections including pharyngitis and tonsillitis.
Management of skin and soft tissue infections such as cellulitis and impetigo.
Therapy for urinary tract infections caused by susceptible organisms.
Treatment of otitis media (middle ear infections) in pediatric patients.
Management of bone infections like osteomyelitis caused by susceptible bacteria.
Prevention and treatment of bacterial endocarditis in dental procedures for at-risk patients.
Used to treat uncomplicated cystitis in women and men.
Therapy for streptococcal infections due to group A beta-hemolytic Streptococcus.
Treatment of dental abscesses and infections of the oral cavity.
Alternative for patients allergic to penicillin with cross-reactivity caution.
Management of infections caused by cephalexin-susceptible strains of Staphylococcus aureus.
Treatment of respiratory tract infections including bronchitis caused by susceptible bacteria.
Used off-label for certain minor infections where beta-lactam sensitive pathogens are implicated.
Part of combination therapy in mixed infections to broaden antimicrobial coverage.
Therapeutic choice in infections where beta-lactamase negative organisms predominate.

Side Or Adverse Effects

Diarrhea is the most commonly reported gastrointestinal side effect.
Nausea and vomiting occurring in some patients, especially if taken on an empty stomach.
Indigestion and abdominal discomfort, usually mild and transient.
Vaginal itching or discharge associated with fungal overgrowth following antibiotic therapy.
Hypersensitivity reactions such as rash, urticaria, and pruritus.
Severe allergic reactions including anaphylaxis are rare but require immediate attention.
Clostridioides difficile-associated diarrhea and colitis, presenting with watery or bloody stools.
Blood dyscrasias such as leukopenia, thrombocytopenia, or eosinophilia in rare cases.
Transient increases in liver enzymes have been documented but are usually reversible.
Headache and dizziness reported rarely during treatment.
Seizures reported in patients with renal impairment or high doses, due to CNS accumulation.
Pain or inflammation at the injection site when using parenteral forms (though not applicable to oral Keflex).
Rare reports of interstitial nephritis leading to renal impairment.
Serum sickness-like reactions presenting with fever, rash, and joint pain have been reported.
Positive Coombs test without hemolytic anemia in select patients.

Precautions

Use with caution in patients with a history of hypersensitivity to beta-lactam antibiotics.
Monitor for signs of allergic reaction, especially in patients with penicillin allergy history.
Exercise caution when prescribing to patients with a history of gastrointestinal diseases, particularly colitis.
Adjust dosage in patients with renal impairment to avoid toxicity.
Avoid unnecessary use to prevent development of antibiotic resistance.
Caution advised in elderly due to age-related renal function decline.
Close monitoring recommended during prolonged therapy for signs of superinfection.
Evaluate renal and hepatic function before and during therapy in at-risk populations.
Use during pregnancy only if clearly needed; classified as pregnancy category B.
Breastfeeding mothers should consult healthcare providers; excreted in breast milk in small amounts.
Avoid concurrent use with nephrotoxic agents without monitoring renal function.
Ensure complete course of therapy to prevent development of resistant strains.
Patients should inform healthcare providers about all allergies to drugs, foods, or dyes.
Observe for early signs of Clostridioides difficile infection in patients with diarrhea.
In patients with mononucleosis, increased risk of rash when taking cephalexin.

Warnings

Do not use in patients with known allergy to cephalexin or other cephalosporins.
Potential cross-reactivity with penicillin allergies; use caution and assess patient history.
Risk of Clostridioides difficile-associated diarrhea which can occur during or after treatment.
Discontinue drug immediately if severe hypersensitivity or skin reactions develop.
Not effective against viral infections such as common cold or influenza.
Prolonged or repeated use may result in fungal or bacterial superinfection.
Renal impairment may lead to accumulation and increased risk of toxicity.
Risk of seizures in patients with severe renal failure receiving high doses.
Careful use recommended in patients with a history of gastrointestinal diseases including colitis.
Allergic reactions may be life-threatening; seek emergency medical care if symptoms appear.
Do not share medication with others; use only for prescribed indication.
Use with caution during pregnancy and lactation; benefits should outweigh risks.
Laboratory tests such as Coombs test, liver function, and renal function may be affected.
Patients should be advised to report any unusual bleeding or bruising promptly.
In case of overdose, seek immediate medical attention owing to possible severe gastrointestinal and neurological effects.

Contraindication

Known hypersensitivity to cephalexin or other cephalosporin antibiotics.
History of allergic reaction to penicillins (cross-sensitivity caution applies).
Patients with previous severe hypersensitivity reactions, including anaphylaxis, to beta-lactams.
Avoid in individuals with history of cholestatic jaundice or hepatic dysfunction related to prior cephalexin use.
Not to be used to treat viral infections or non-bacterial etiologies.
Contraindicated in patients with known active gastrointestinal colitis associated with broad-spectrum antibiotics.
Avoid use in neonates and infants younger than one year due to insufficient safety data.
Do not administer in patients with hypersensitivity to any of the capsule or tablet excipients if applicable.

Drug Interactions

Probenecid can decrease renal tubular secretion of cephalexin, leading to increased plasma concentrations.
Concomitant administration with metformin requires monitoring as renal function changes may affect metformin clearance.
May interfere with urinary glucose tests producing false-positive results due to beta-lactam structure.
Combination with other nephrotoxic drugs (e.g., aminoglycosides, loop diuretics) may increase risk of kidney damage.
Use with oral anticoagulants may potentiate bleeding risk; monitor coagulation parameters.
Possible antagonism with bacteriostatic antibiotics like tetracyclines which inhibit bacterial growth.
Concurrent use with live bacterial vaccines may reduce vaccine efficacy.
No significant interactions reported with cytochrome P450 metabolized drugs due to minimal hepatic metabolism.
Antacids and H2 blockers do not significantly alter absorption but may affect gastrointestinal tolerance.
Interactions with oral contraceptives are unlikely; however, patients should be advised of potential decreased efficacy with antibiotics in general.
May interfere with some laboratory serologic tests, including Coombs tests, leading to false-positive results.
Careful monitoring advised when combined with other renally cleared medications to prevent accumulation.
Concurrent use with NSAIDs generally safe but monitor for additive renal effects.
Herbal products or supplements with nephrotoxic potential should be used cautiously.
Alcohol does not have a direct interaction but may exacerbate side effects such as gastrointestinal upset.

Preparations Of The Drug

Oral capsules available in 250 mg, 500 mg, and 750 mg strengths for the treatment of various infections.
Oral tablets supplied in 250 mg and 500 mg doses intended for easy administration in adult and adolescent patients.
Oral suspension formulations provided as 125 mg/5 mL and 250 mg/5 mL concentrations to facilitate pediatric dosing and ease of swallowing.
Formulations manufactured to comply with stringent quality control standards ensuring potency and safety.
Packaging typically includes child-resistant caps to prevent accidental ingestion by children.
Bulk powders for oral suspension requiring reconstitution prior to dispensing to ensure correct dosing.
Film-coated tablets designed to improve palatability and ease of swallowing.
Generic formulations are widely available, ensuring accessibility and cost-effectiveness.
Stability of oral liquid preparations maintained for specified durations when stored properly at room temperature.
Ready-to-dispense unit dose blister packs available for outpatient prescription use.
Specialty compounded formulations may be prepared for patients with specific dosage requirements or allergies to excipients.
Standard formulations intended for both inpatient hospital use and outpatient therapy.
Storage conditions recommended at controlled room temperature, protecting from moisture and heat to preserve drug integrity.
Non-prescription combination products with cephalexin are not available; all formulations are prescription-only.
All preparations are intended for oral use only; parenteral cephalosporins are distinct and not interchangeable.