Thromboembolism and oral contraceptives. (Letter to the editor).

The article by Drill and Calhoun (219: 583, 1972) and subsequent comments (220: 416, 1972, and 221: 194, 1972) on the problem of thromboembolic episodes and oral contraceptives consider at length the problems of retrospective and prospective studies. If the critical standards presented had been applied to the articles cited by Drill and Calhoun, there would not have been much left to discuss. The citations of Goldzieher and of Tyler, for example, are not to research reports but to short sections of articles of a very general nature. Tyler's comments are not even quantitative. 10 of the 15 articles cited by Drill and Calhoun in their principle tabulation (Table 3) were readily available to me. When I arranged these articles in order of scientific merit as regards the embolism problem, it became apparent that the better the quality of the scientific effort, the higher was the reported incidence of thromboembolism and the lower the number of subjects. It is very possible that the inclusion of a few casual reports with large numbers of subjects is producing an erroneous, low, overall incidence of thromboembolic episodes.