Group: antimycobacterial agent
Tablet 100-400 mg (hydrochloride)
A synthetic congener of 1,2-ethanediamine that is bactericidal against M. tuberculosis, M. bovis and some nonspecific mycobacteria. It is used in combination with other antituberculosis drugs to prevent or delay the emergence of resistant strains.
It is readily absorbed from the gastrointestinal tract. Plasma concentrations peak in 2-4 hours and decay with a half-life of 3-4 hours. Ethambutol is excreted in the urine both unchanged and as inactive hepatic metabolites. About 20% is excreted in the faeces as unchanged drug.
An optional component of several combined antituberculosis chemotherapeutic regimens currently recommended by WHO (see pages 12 and 13), of particular use when primary resistance to other drugs is suspected.
Dosage and administration
Adults: 25 mg/kg daily for no more than 2 months followed, as appropriate, by 15 mg/kg daily; or 40 mg/kg three times weekly.
Children: 15 mg/kg daily.
Dosage must always be carefully calculated on a weight basis to avoid toxicity, and should be reduced in patients with impaired renal function.
• Known hypersensitivity.
• Pre-existing optic neuritis from any cause.
• Inability (for example due to young age) to report symptomatic visual disturbances.
• Creatinine clearance of less than 50 ml/ minute.
Patients should be advised to discontinue treatment immediately and to report to a doctor should their sight or perception of colour deteriorate. Patients who are too young or who are otherwise unable to comprehend this warning should not receive ethambutol.
Whenever possible, renal function should be assessed before treatment.
Use in pregnancy
Where there is no evidence of primary resistance, the 6-month regimen based upon isoniazid, rifampicin and pyrazinamide should be used. If a fourth drug is needed during the initial phase, ethambutol should be preferred to streptomycin.
Dose-dependent optic neuritis can readily result in impairment of visual acuity and colour vision. Early changes are usually reversible, but blindness can occur if treatment is not discontinued promptly.
Signs of peripheral neuritis occasionally develop in the legs.
Emesis and gastric lavage may be of value if undertaken within a few hours of ingestion. Subsequently, dialysis may be of value. There is no specific antidote and treatment is supportive.
Tablets should be stored in well-closed containers.