If thioacetazone is replaced by ethambutol, patients with initial isoniazid resistance failing on treatment will have acquired ethambutol resistance.  The re-treatment regimen provides now effective rifampicin monotherapy in the continuation phase.  Because of the low frequency of mutations that confer resistance to rifampicin and the usually low bacillary load that is expected after the intensive phase, this may not carry a great danger but the potential certainly exists as will be shown.

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