ClinCalc Pro
Menu
Calculators Drugs Pathways AI Dx Donate Get the App
Macrolide Antibiotic Pregnancy: Compatible — azithromycin safe in pregnancy; used for chlamydia treatment

Azithromycin

Brand names: Zithromax

Adult dose

Dose: Community-acquired pneumonia: 500 mg OD for 3–5 days. STI (chlamydia): 1 g single dose. MAC prophylaxis (HIV): 1.25 g once weekly.
Route: Oral or IV
Frequency: OD (3–5 days typical), or single dose
Max: 500 mg/day
Long tissue half-life (68h) — 3-day course equivalent to 10-day amoxicillin in soft tissue infections. QT prolongation — avoid in patients with known QT prolongation or on QT-prolonging drugs.

Paediatric dose

Dose: 10 mg/kg
Route: Oral
Frequency: Once daily (3-day course)
Max: 500 mg/day
Concentration: 40 mg/ml
BNF for Children: 10 mg/kg OD × 3 days (max 500 mg). Neonatal chlamydial conjunctivitis: 20 mg/kg OD × 3 days. Pertussis: 10 mg/kg OD × 5 days. Avoid <6 weeks (hypertrophic pyloric stenosis risk). Source: BNF for Children 2024

Dose adjustments

Renal

No dose adjustment required

Hepatic

Avoid in severe hepatic impairment — hepatically metabolised and excreted

Paediatric weight-based calculator

BNF for Children: 10 mg/kg OD × 3 days (max 500 mg). Neonatal chlamydial conjunctivitis: 20 mg/kg OD × 3 days. Pertussis: 10 mg/kg OD × 5 days. Avoid <6 weeks (hypertrophic pyloric stenosis risk). Source: BNF for Children 2024

Clinical pearls

  • Long tissue half-life: do not give repeated short courses; tissue drug persists for 5–7 days after last dose
  • COVID-19: widely used in early pandemic but RECOVERY trial showed NO benefit for COVID-19
  • Chlamydia treatment: 1 g single dose (azithromycin) or doxycycline 100 mg BD × 7 days — both effective (BASHH guidelines)
  • QTc prolongation: check baseline ECG in at-risk patients before prescribing

Contraindications

  • Congenital or acquired QT prolongation
  • Hypokalaemia or hypomagnesaemia (QT risk)
  • Concurrent QT-prolonging drugs

Side effects

  • GI upset (nausea, diarrhoea)
  • QT prolongation
  • Tinnitus and hearing loss (rare — high doses)
  • Cholestatic jaundice (rare)
  • Anaphylaxis (rare)

Interactions

  • QT-prolonging drugs — additive QT prolongation
  • Warfarin — may slightly increase INR
  • Antacids (aluminium/magnesium) — reduce azithromycin absorption (separate by 2h)
  • Digoxin — increased digoxin levels (P-gp inhibition)

Monitoring

  • ECG (QTc) if at risk
  • GI tolerability
  • Clinical response
  • LFTs if prolonged use

Reference: BNFc; BNF; BASHH STI Guidelines; RECOVERY Trial (Horby et al, NEJM 2021). Verify against your local formulary and the latest BNF before prescribing.

Related

Curated clinical cross-links plus same-class fallbacks.

Calculators
Drugs
Pathways