Lasting effects of oral contraceptives on fear responses to a safe context.

Estradiol (E2), whether endogenous or administered via oral contraceptives (OCs), modulates fear regulation. However, its role in contextual fear signaling, critical for distinguishing threat from safety, remains poorly understood in humans. While prior human studies focused on cue-related extinction recall, animal research suggests that a single high-dose of exogenous E2 generalizes fear to safe contexts. This study tested whether hormonal status and OC use influence contextual fear responses in humans, and whether OC-related effects are long-lasting and dose-dependent. In a two-day fear conditioning and extinction protocol, 147 healthy participants (men, cycling women, OC users) underwent fear conditioning in a threat-associated context (CX + ) and extinction in a safety-associated context (CX-). On day 2, cues were presented in both contexts. Fear return to each context was assessed via skin conductance responses and brain activations. Groups were formed sequentially using the same participants: (A) by endogenous/exogenous E2 status, (B) by OC use history, and (C) by current ethinyl estradiol (EE) dose. CX- fear returns were elevated in current (p = .015, d = 0.64) and past OC users (p = .014, d = 0.83) compared to never users. High-EE, but not low-EE users, showed greater fear returns than never users (p = .021, d = 0.81). Biological sex and endogenous E2 were unrelated to contextual fear. Across participants, CX- fear returns were negatively associated with hippocampal and ventromedial prefrontal cortex activation. OC use, particularly at higher EE doses, may impair retrieval of safety signals from context, with effects persisting beyond discontinuation. These findings highlight exogenous hormones as a modulator of contextual fear regulation in women.