OC, depressive and insomnia symptoms in adult women with and without depression 59 randomized or assigned, which means OC use during the study purely relied on participant’s own contraceptive use choice. The choice to use or not use OC can depend on medical history or comorbidities, as seen in for example, medical advice to not use OC in women with history of thrombosis, or to use OC in women with polycystic ovary syndrome or endometriosis, as well as on personal experiences using OC: women who had adverse experiences using OC were found to be less likely to continue OC use (Westhoff et al., 2007). This implies the distribution of OC use across the participants was most likely not random. Following this reasoning, it would mean that measurements in women in our study who experience less adverse mood during OC use could be overrepresented in the OC measurements, whereas measurements in women who do experience more adverse mood effects could be overrepresented in either NC measurements or in the group of measurements in women who “switched” OC use during the study (e.g. the group of participants who contributed most to the within-subject estimates). This form of selection bias, specifically the idea that women who experience adverse effects during OC use are less likely to be present in long-term users of OC, has previously been deemed the “healthy survivor effect” (Zettermark et al., 2018). Based on this idea of the “healthy survivor effect”, our findings do seem to follow a pattern that was previously found in the literature: studies which compared population-based groups of OC users with groups of non-users found OC to be associated lower depression prevalence (Keyes et al., 2013; Toffol et al., 2011), whereas studies that have prospectively assessed OC use over time have found adverse effects of OC on depression diagnoses and symptoms (Skovlund et al., 2016). Within the context of this previous work, our findings accentuate some important points. First, it could be that OC negatively affects depression symptoms in some women, who might then discontinue their OC use, while other women remain unaffected. Previous work has already highlighted possible mechanisms involving progesterone and GABA receptors, in which there are strong individual differences in sensitivity to sex hormone-associated mood symptoms (Schweizer-Schubert et al., 2021). Secondly, this highlights the importance of within-person comparisons in the field of sex hormones and mood. This point has been raised before in menstrual cycle research (Schmalenberger et al., 2021) and our findings highlight the relevance of accounting for within- and betweenparticipant comparisons.
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