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Antituberculous Agents Pregnancy: Use with caution — ethambutol crosses placenta; WHO recommends use in pregnancy for active TB; no proven teratogenicity; visual monitoring particularly important as pregnancy can affect visual acuity

Ethambutol

Brand names: Myambutol

Adult dose

Dose: 15 mg/kg once daily (standard initial phase)
Route: Oral
Frequency: Once daily
Max: 1.5 g/day (some guidelines allow up to 2.5 g based on weight)
Used during the initial 2-month phase of RIPE only — stopped after 2 months if drug sensitivities confirm fully sensitive TB. Take with food to reduce GI upset. VISUAL ACUITY AND COLOUR VISION TESTING MANDATORY before starting.

Paediatric dose

Dose: 20 mg/kg once daily mg/kg
Route: Oral
Frequency: Once daily
Max: 1 g/day
Ethambutol avoided in children too young to reliably report visual symptoms (typically under 5 years) — use specialist paediatric guidance; visual acuity testing required

Dose adjustments

Renal

Significant dose reduction required: eGFR 30–60 → 15 mg/kg every 36 hours; eGFR below 30 → 15 mg/kg every 48 hours; haemodialysis → post-dialysis dose; renal failure greatly increases optic neuritis risk

Hepatic

No specific adjustment required

Paediatric weight-based calculator

Ethambutol avoided in children too young to reliably report visual symptoms (typically under 5 years) — use specialist paediatric guidance; visual acuity testing required

Clinical pearls

  • OPTIC NEURITIS — MOST CRITICAL CONCERN: ethambutol is used in initial phase to protect against emergent isoniazid resistance; once drug susceptibilities confirm sensitivity, ethambutol is STOPPED — usually at 2 months; risk of optic neuritis is dose and duration related
  • VISUAL MONITORING MANDATORY: baseline visual acuity (Snellen chart) AND colour vision (Ishihara plates or D-15 Farnsworth test) before starting; monthly visual assessment during treatment; instruct patient to stop ethambutol IMMEDIATELY and report if vision changes
  • RED-GREEN COLOUR VISION LOSS: the earliest and most sensitive sign of ethambutol optic neuritis — precedes visual acuity loss; test colour vision monthly with Ishihara plates; any deterioration requires immediate specialist ophthalmology referral and drug discontinuation
  • RENAL DOSE ADJUSTMENT CRITICAL: ethambutol accumulates in renal failure; optic neuritis risk dramatically increased in CKD patients given standard doses; always calculate eGFR and adjust dose before prescribing — this is a key MHRA safety concern
  • Role in TB treatment: ethambutol prevents emergence of rifampicin resistance during initial phase when drug susceptibilities are unknown; once full sensitivity confirmed, can discontinue; used throughout if isoniazid or rifampicin resistance present
  • MHRA / NICE NG33: ethambutol is part of standard 4-drug RIPE initial phase; visual acuity monitoring is mandatory throughout ethambutol use

Contraindications

  • Pre-existing optic neuritis
  • Unable to perform visual acuity monitoring (children under 5, severe visual impairment)
  • Known hypersensitivity to ethambutol

Side effects

  • Optic neuritis — MOST SERIOUS: dose-dependent; onset usually after 2+ months; presents as decreased visual acuity, colour vision loss (red-green — most sensitive early sign), central scotoma; USUALLY REVERSIBLE IF STOPPED EARLY; rarely irreversible blindness
  • Hyperuricaemia and gout (less than pyrazinamide)
  • Peripheral neuropathy (rare)
  • Nausea, abdominal discomfort

Interactions

  • Antacids containing aluminium hydroxide (reduce ethambutol absorption — separate by 4 hours)
  • No significant CYP450 interactions

Monitoring

  • Visual acuity (baseline and monthly during treatment — Snellen chart)
  • Colour vision (baseline and monthly — Ishihara plates or D-15 test)
  • Renal function (eGFR — dose adjustment threshold)
  • Serum urate
  • Monthly sputum smear and culture

Reference: BNFc; BNF 90; NICE NG33; WHO TB Guidelines 2022; BTS TB Guidelines; MHRA SPC Myambutol. Verify against your local formulary and the latest BNF before prescribing.

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Curated clinical cross-links plus same-class fallbacks.

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