Drug resistance in out-patients treated with sodium PAS and isoniazid cachets

Summary The development of isoniazid resistance accompanied by radiographic or bacteriological evidence of relapse has been investigated in a series of 12 out-patients prescribed apparently adequate combinations of sodium PAS and isoniazid dispensed in the same cachet. The emergence of drug resistance was believed to be due to irregular ingestion of the cachets. If relapse, for which there is no other satisfactory explanation, occurs during treatment with sodium PAS and isoniazid in adequate dosages, the drugs should be discontinued temperarily until sensitivity results are obtained. Otherwise loss of sensitivity to PAS may also develop, as occurred in 4 of the 12 individuals in this series.