Differential effects of hormone therapy formulations on Parkinson's disease risk: a systematic review and meta-analysis.

BACKGROUND

The relationship between menopausal hormone therapy (HT) and Parkinson's disease (PD) risk remains controversial, with inconsistent findings potentially driven by differences in hormonal formulations.

METHODS

We conducted a systematic review and meta-analysis following PRISMA 2020 guidelines. MEDLINE/PubMed and EMBASE were searched between January 8 and March 14, 2026. Observational studies evaluating the association between menopausal HT and PD risk were included. Effect estimates were pooled using random-effects models. Subgroup analyses were performed according to HT formulation (estrogen-only vs. combined estrogen-progestin therapy). A multilevel meta-analysis was conducted to account for within-study dependence.

RESULTS

Fourteen studies including 753,749 participants (4,433 PD cases) were analyzed. Overall, HT was not significantly associated with PD risk (RR 1.08; 95% CI 0.94-1.23; I² = 49.4%). In subgroup analyses, combined therapy was associated with an increased PD risk (RR 1.40; 95% CI 1.07-1.82), whereas estrogen-only therapy showed no significant association (RR 1.01; 95% CI 0.81-1.27). Multilevel analysis yielded consistent results (combined: RR 1.33; 95% CI 1.00-1.77; estrogen-only: RR 1.03; 95% CI 0.83-1.27), with no statistically significant interaction between formulations (p = 0.12).

CONCLUSIONS

Combined menopausal HT was associated with an increased risk of PD, while estrogen-only therapy showed no significant association. Although differences between formulations were not statistically significant, these findings suggest that hormone composition may influence neurological outcomes and warrant further investigation into individualized HT strategies.