An assessment of the carcinogenicity of isoniazid in patients with pulmonary tuberculosis.

In an assessment of the carcinogenicity of isoniazid, 3,842 adult tuberculous patients admitted to 2 sanatoria in the period 1950 to 1957 (that is, in the years immediately before and after the introduction of isoniazid) have been followed up for a mean period of over 19 years. Their mortality has been compared with that expected during the same calendar period in a general population group in England and Wales with the same age and sex distribution. The relative risk of death (the observed divided by the expected number of deaths) from all malignant neoplasms in patients first starting chemotherapy in 1950 to 1952, before the general introduction of isoniazid, was 0.8 for those who received isoniazid at some time, compared with 0.5 for those who never received it; for those first starting chemotherapy in 1953 to 1957, after the general introduction of the drug, the respective risks were 1.4 and 1.8. The relative risk of death from malignant neoplasms was 2.1 in the first 4 years after starting the treatment with isoniazid; this high relative risk is unlikely to be attributable to isoniazid and largely disappears subsequently, for in successive 4-year periods it was 1.3, 0.9, 1.2 and 1.4. The relative risks of death from all malignant neoplasms for patients receiving a total dosage of less than 50, 50-99, 100-199 and 200 g or more were 1.5, 1.5, 1.0 and 1.3, respectively. For patients receiving a maximum daily dose of less than 250 g the relative risk was 1.3, and for those receiving 250 g or more it was 1.2. There was a curious and unexplained difference in the mortality from malignant neoplasms in patients first starting chemotherapy in 1950 to 1952 (relative risk 0.6) and those first starting in 1953 to 1957 (relative risk 1.5). This is being studied further. This study has provided no evidence of a carcinogenic effect of isoniazid in a period of follow-up averaging nearly 20 years. The follow-up is being continued.