Generic Name

Cephalexin

Mechanism

• Inhibits bacterial cell‑wall synthesis by binding to penicillin‑binding proteins (PBPs) on the cell membrane.
• Binding blocks transpeptidation, preventing peptidoglycan cross‑linking → bacterial lysis.
• Primarily active against gram‑positive cocci (e.g., *Staphylococcus aureus*, *Streptococcus pyogenes*, *S. pneumoniae*) and selective gram‑negative rods (*E. coli*, *Proteus vulgaris*).

Pharmacokinetics

• Absorption: 65‑90 % oral bioavailability; peak plasma 1–2 h post‑dose.
• Distribution: 35‑41 % plasma protein binding; distributes into skin, soft tissue, urine, bronchial alveolar fluid.
• Metabolism: Largely unchanged; minimal hepatic metabolism.
• Elimination: Renally excreted (≈ 90 % unchanged). Half‑life ≈ 0.7–1 h (normal renal function).
• Adjustments: In renal impairment, dosing interval or amount is reduced based on GFR.

Indications

• Skin and soft‑tissue infections: cellulitis, abscesses, impetigo
• Respiratory tract infections: strep tonsillitis, pharyngitis, sinusitis (suspected gram‑positive organisms)
• Urinary tract infections: cystitis, urethritis, pyelonephritis when susceptible
• Bone and joint infections: osteomyelitis, septic arthritis
• Other: dental abscesses, postoperative prophylaxis for certain procedures

Contraindications

• IgE‑mediated hypersensitivity to cephalosporins or first‑generation cephalosporins in patients with a history of penicillin allergy (cross‑reactivity ≈ 10 %).
• Severe renal dysfunction: avoid or use extreme caution.
• Pregnancy: Category B – use only if benefits outweigh risks.
• Lactation: excreted in breast milk; caution advised.
• Concurrent use with tetracyclines: risk of altered absorption – avoid unless no alternative.
• Concurrent NSAIDs: may increase renal toxicity.

Dosing

• Route: Oral tablets or liquid suspension.
• Food interaction: Food does not significantly affect absorption; may be taken with or without food to improve tolerability.

Adverse Effects

• GI: nausea, vomiting, diarrhea, abdominal pain (most frequent).
• Allergic reactions: rash, pruritus, urticaria; severe: anaphylaxis, Stevens‑Johnson syndrome.
• Metabolic: hypokalemia (rare), hyperuricaemia.
• Hematologic: neutropenia, thrombocytopenia (often reversible).
• Superinfections: *Clostridioides difficile* colitis (rare).
• Renal: acute interstitial nephritis (very rare).

Monitoring

• Renal function: serum creatinine, eGFR at baseline; repeat if dosing > 3 weeks or at dose changes.
• CBC: for prolonged therapy (> 14 days) due to neutropenia risk.
• Signs of severe infection: worsening fever, chills, hypotension → switch to broader coverage if no response.
• Allergic history: monitor for rash or hypersensitivity reactions, especially during the first 24 h.

Clinical Pearls

• Use the lowest effective dose for the shortest duration to curb resistance; discontinue after 7–10 days for uncomplicated infections.
• Renal dosing calculators are essential—especially in elderly or dialysis patients—to avoid toxicity.
• Cephalexin retains potency against penicillin‑susceptible MRSA (PSSA) but not MRSA‑resistant strains; use synergy with clindamycin or linezolid for MRSA coverage.
• Concurrent NSAIDs (e.g., ibuprofen) should be avoided in renal impairment due to additive nephrotoxicity.
• Rapid transition from IV to oral is supported by pharmacokinetics: achieve comparable serum levels 1 h after oral dose.
• Always ensure your photonicity: some rural areas have started to demonstrate resistance in *E. coli*—culture/susceptibility testing is prudent for UTIs with unclear etiology.
• Pregnancy & lactation: although Category B, minimize use unless benefit outweighs minimal risk; consider alternative beta‑lactams if maternal allergy is suspected.

Key Takeaway: Cephalexin’s efficacy, safety, and oral bioavailability make it a first‑line agent for many community‑acquired infections, provided dosing is adjusted for renal function and patient allergy status.