Introduction
Over recent years, three Nordic large-scale epidemiological studies have tried to quantify the influence of use of different types of hormonal contraceptives on the risk of developing depression or use of antidepressant drugs1-3. In the following, the first Danish study1 is called “study 1”, the first Swedish study2 “study 2”, and the second Swedish study3 “study 3” according to time of publication. Although at first sight these three studies seem rather similar in methods, the authors of these three studies – surprisingly - made quite different conclusions from their findings, although some results were similar in all three studies. Going into details on the methods, however, may reveal differences which might explain the different findings and conclusions.
The issue is important, because hormonal contraception (HC) is one of the most frequent medical exposures worldwide, and depression or use of antidepressants are among the most frequent outcomes in the same countries.
The aim of this study was to compare the three studies and explore whether differences in methods may explain the different results, and whether ideological differences towards hormonal contraception also may play a role for the chosen methods and the interpretation of the results.