Screening for possible drug carcinogenicity: second report of findings.

Cancer development was followed up through 1978 in 143,574 outpatients who were identified on the basis of computer-stored drug-dispensing data as users of various medicinal drugs during the 4-year period starting in July 1969. Screening analyses were repeated for the 95 most commonly used drugs and drug groups, which were reported previously after a shorter follow-up, and it was found that several drug-cancer associations had either gained or lost statistical significance since the report. Screening of 120 additional less commonly used drugs revealed one negative and 43 positive drug-cancer associations. A few of these associations suggested interesting new hypotheses about possible drug effects, but further study is required before conclusions on causality can be drawn. The absence of association with cancer for many drugs provides some provisional assurance for the absence of carcinogenicity, but further follow-up is needed to rule out later effects.