Ethambutol dosage

In patients with impaired glomerular filtration or any other renal dysfunction, ethambutol dosages have to be altered (25). 

Optic neuritis (more common with high-dosage or long-term ethambutol therapy), peripheral neuritis, thrombocytopenia, and an anaphylactoid reaction occur rarely. 

Hypersensitivity to ethambutol is rare. The most common serious adverse event is retrobulbar neuritis, resulting in loss of visual acuity and red-green color blindness. This dose-related adverse effect is more likely to occur at dosages of 25 mg/kg/d continued for several months. At 15 mg/kg/d or less, visual disturbances are very rare. Periodic visual acuity testing is desirable if the 25 mg/kg/d dosage is used. Ethambutol is relatively contraindicated in children too young to permit assessment of visual acuity and red-green color discrimination. 

The adverse effects of ethambutol are mainly seen in patients taking very high doses, that is over 25 mg/kg/day. Dosages over 25 mg/kg/day should be administered for only about 2 weeks at the beginning of therapy. Treatment can then be continued with 15 mg/kg/day. If initial problems arise and ethambutol is essential, the daily dose should not exceed 10 mg/kg. 

Ocular disturbances due to ethambutol are dose-related. Dosages not over 25 mg/kg/day during the first 2 months of treatment and 15 mg/kg/day thereafter are generally accepted as adequate (7). At a dosage of 15 mg/kg/day, which should be regarded as a maximum for maintenance therapy, ocular toxicity developed in only 1.6% of patients (8). Advanced age (9), renal insufficiency, and diabetes can enhance ocular damage. 

Strauss I, Erhardt F. Ethambutol absorption, excretion and dosage in patients with renal tuberculosis. Chemotherapy 1970 15(3) 148-57. 

Ethambutol is available for oral administration in tablets containing the D isomer. The usual adult dose of ethambutol is 15 mg/kg given once a day. Some physicians prefer to treat with 25 mg/kg per day for the first 60 days and then to reduce the dose to 15 mg/kg per day, particularly for those who have received previous therapy. Ethambutol accumulates in patients with impaired renal function and adjustment of dosage is necessary. 

Konno et al. (1968) saw three cases of drug exanthema among 81 patients treated with ethambutol. One patient was successfully desensitized, starting from small doses of ethambutol, and the full dosage could eventually be given again. 

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