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Macrolide antibiotic Pregnancy: Avoid if possible — animal studies show embryotoxicity at high doses; limited human data. Use erythromycin or azithromycin as safer macrolide alternatives in pregnancy.

Clarithromycin 500mg (Atypical CAP)

Brand names: Klaricid (250 mg and 500 mg tablets), Klaricid XL (500 mg modified release)

Adult dose

Dose: 500 mg twice daily for 5–7 days
Route: Oral (standard or modified release)
Frequency: Twice daily (500 mg BD) or Klaricid XL 500 mg once daily (MR formulation)
Max: 1 g/day (standard); 500 mg OD (XL formulation)
Community-acquired pneumonia (atypical — Mycoplasma pneumoniae, Legionella pneumophila, Chlamydophila pneumoniae): 500 mg BD for 5–7 days. Add to beta-lactam (amoxicillin/co-amoxiclav) for dual coverage in moderate-severe CAP. Klaricid XL (MR formulation): once daily dosing — improves adherence. IV clarithromycin (500 mg BD) for hospitalised patients unable to take oral.

Paediatric dose

Dose: 7.5 mg/kg
Route: Oral
Frequency: Twice daily
Max: 500 mg per dose
Concentration: 125 mg/5 mL or 250 mg/5 mL suspension mg/ml
Children: 7.5 mg/kg BD (max 500 mg BD). Suspension available. For atypical pneumonia in children ≥3 months.

Dose adjustments

Renal

eGFR <30 mL/min: halve the dose or double the dosing interval.

Hepatic

Use with caution in hepatic impairment — hepatically metabolised. Avoid in severe hepatic impairment.

Paediatric weight-based calculator

Children: 7.5 mg/kg BD (max 500 mg BD). Suspension available. For atypical pneumonia in children ≥3 months.

Clinical pearls

  • Stop statins during clarithromycin course: CYP3A4 inhibition dramatically increases statin exposure — risk of myopathy/rhabdomyolysis with simvastatin and atorvastatin
  • Dual therapy for CAP: NICE NG120 recommends beta-lactam + macrolide for moderate-severe CAP (CRB-65 ≥2) for atypical coverage — clarithromycin added to co-amoxiclav IV
  • BTS CAP severity score: use CURB-65 (Confusion, Urea >7, RR ≥30, BP <90 or ≤60, Age ≥65) to guide outpatient vs inpatient and mono vs dual therapy
  • Metallic taste: almost universal — warn patients in advance; can be severe enough to cause adherence issues
  • IV to oral switch: clarithromycin 500 mg BD IV to oral within 24–48h when clinically improving and tolerating oral fluids — equivalent bioavailability

Contraindications

  • History of QT prolongation or torsades de pointes
  • Concomitant drugs that prolong QT interval (see interactions)
  • Hypersensitivity to macrolides

Side effects

  • GI disturbance (nausea, diarrhoea, abdominal pain — most common)
  • Metallic or bitter taste
  • QT prolongation (more significant than azithromycin)
  • Elevated LFTs / hepatotoxicity (rare)
  • Drug interactions (multiple CYP3A4 interactions — see below)

Interactions

  • Statins (simvastatin, atorvastatin) — markedly increased statin levels; hold statin during clarithromycin course (myopathy/rhabdomyolysis)
  • Warfarin — significantly increases INR; monitor daily during course
  • Colchicine — avoid combination (fatal colchicine toxicity reported)
  • QT-prolonging drugs (antipsychotics, amiodarone, fluoroquinolones) — additive QT prolongation
  • Theophylline/aminophylline — increases theophylline levels by 20–40%; reduce dose or monitor levels
  • Ciclosporin/tacrolimus — increased immunosuppressant levels; monitor

Monitoring

  • Clinical response at 48–72 hours
  • ECG if pre-existing cardiac risk or QT-prolonging drugs
  • LFTs if prolonged course
  • Statin myopathy symptoms

Reference: BNFc; BNF; NICE NG120 Pneumonia; NICE CKS LRTI; BTS CAP Guidelines 2009 (updated). Verify against your local formulary and the latest BNF before prescribing.

Related

Curated clinical cross-links plus same-class fallbacks.

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