Antitubercular — First-Line TB (RIPE Regimen, 'E') Pregnancy: Use with caution — animal studies show teratogenicity at high doses; used in pregnancy when benefit outweighs risk (TB in pregnancy requires treatment)

Ethambutol

Brand names: Myambutol

Adult dose

Dose: 15 mg/kg once daily (initial phase); intermittent regimen: 30 mg/kg 3x/week

Route: Oral

Frequency: Once daily

Max: 2.5 g/day

Part of standard RIPE (rifampicin, isoniazid, pyrazinamide, ethambutol) 6-month TB regimen — all four given for 2 months initial phase; rifampicin + isoniazid continued for 4 months continuation phase. MANDATORY: visual acuity and colour vision tested before starting; repeated monthly. Weight-based dosing — recalculate as weight changes during treatment.

Paediatric dose

Dose: 20 mg/kg mg/kg

Route: Oral

Frequency: Once daily

Max: 2.5 g/day

BNFc: use with caution in children too young to report visual symptoms — colour vision testing challenging; NICE recommends only if culture confirmed TB or drug resistance suspected

Dose adjustments

Renal

CrCl <30 mL/min: reduce to 15–25 mg/kg 3x/week; haemodialysis — dose post-dialysis

Hepatic

No dose adjustment required

Paediatric weight-based calculator

Patient weight (kg)

BNFc: use with caution in children too young to report visual symptoms — colour vision testing challenging; NICE recommends only if culture confirmed TB or drug resistance suspected

Clinical pearls

Optic neuritis is the key toxicity — classically causes loss of red-green colour discrimination before visual acuity drops; monthly Ishihara plates and Snellen chart mandatory; stop immediately if vision deteriorates — usually reversible if caught early
RIPE mnemonic: Rifampicin (hepatotoxicity, red secretions, enzyme inducer), Isoniazid (peripheral neuropathy — give pyridoxine), Pyrazinamide (hepatotoxicity, gout), Ethambutol (optic neuritis)
Not used in smear-negative non-respiratory TB where drug resistance unlikely — 3-drug regimen (RIP) sufficient
Ethambutol blocks arabinosyl transferase — inhibits arabinogalactan synthesis (mycobacterial cell wall)

Contraindications

Pre-existing optic neuritis
Unable to report visual symptoms (young children, cognitive impairment)
Hypersensitivity to ethambutol

Side effects

Optic neuritis (dose-dependent — reduced visual acuity, colour vision loss — especially red-green; usually reversible if detected early)
Peripheral neuropathy
Hyperuricaemia (less than pyrazinamide)
Rash
GI disturbance

Interactions

Antacids (aluminium salts) — reduce ethambutol absorption (separate by 4 hours)
Other optic-toxic drugs — additive risk

Monitoring

Visual acuity monthly (Snellen chart)
Colour vision monthly (Ishihara plates)
Renal function (dose adjustment)
Uric acid
LFTs (TB drug hepatotoxicity)

Reference: BNFc; BNF 90; NICE NG33 (TB Management); PHE TB Treatment Guidelines; WHO TB Treatment Guidelines 2022. Verify against your local formulary and the latest BNF before prescribing.

Curated clinical cross-links plus same-class fallbacks.

Calculators

Pathways