Exclusions
Previous studies have demonstrated former depression to be the most important risk factor for new depressions. Therefore, it is correct primarily to exclude women with previous depression from such studies. It makes no sense, however, to disregard the first four weeks of use of HC and decide to classify this exposure period as non-exposed. Those who experience mood changes by use of HC often report these symptoms within days after starting this use, as demonstrated in study 1 and 2 which followed women from start of HC exposure. That methodological decision in study 3 will certainly underestimate the real risk of depression development.
The other main difference between study 1 and 2 versus study 3 was the exclusion of all women having used HC before turning 15 years. Thereby, the age groups with the highest sensitivity for depression development with HC use were excluded. Many of these women would have started use of antidepressants after turning 15 years, while still being exposed to HC. Exclusion of this group will thus also underestimate the risk of depression development.
Finally, women who in addition to a depression also were recorded with an anxiety diagnosis or use of anxiolytics at the same time as being prescribed antidepressants, were excluded in study 3, also a circumstance which would diminish the risk of depression development, as this condition, especially in young women, is often associated with anxiety disorders.
These three methodological circumstances are likely the main reason for the discrepancies in results between study 1 and 2 versus study 3.