The last four decades have seen spectacular developments in the management of pulmonary tuberculosis. The discovery of streptomycin in 1943 by Waksman was soon followed by other drugs with anti-tuberculosis activity, such as PAS, isoniazid, pyrazinamide, thioacetazone, ethambutol, rifampicin and others. The dis-covery of the anti-tuberculosis activity of isonia-zid in 1952 was a great land-mark which re-volutionalised the treatment of tuberculosis. Because of its high efficacy, low toxicity and low cost, isoniazid soon became the drug of choice in tuberculosis. Administered alone daily for 12 months, it has a potential of producing bacteri-ological quiescence in approximately 70% of pulmonary tuberculosis cases excreting isoniazid-sensitive cultures in their sputum. The con-comitant administration of thioacetazone or PAS daily for 1 year increases the efficacy of the regimen to about 85%. However, approximately 20% of the patients with quiescent disease at 1 year have a bacteriological relapse after stopping chemotherapy, so that the overall efficacy of the 2-drug regimens is reduced to less than 70% even among those patients who consume the drugs regularly. Such relapses can be prevented by continuing chemotherapy beyond 12 months for another 6-12 months. It is, therefore, customary to prescribe regimens of 8-24 months to tuberculosis patients
