Objective To assess the protective effect of depot‐medroxyprogesterone acetate (DMPA) on uterine leiomyomas. DMPA has been widely used in Thailand for many years; uterine leiomyomas is the most common female tumour. Design A multicentre hospital‐based case‐control study. Setting University and regional hospitals. Patients Cases were all newly diagnosed patients with pathologically proven diagnosis of uterine leiomyomas, who were admitted to eight hospitals in three regions of Thailand from January 1991; to June 1993;. Three controls matched with cases by sex, age within five years and date of admission within three months were recruited. Main outcome measures Information on socio‐demographic factors, personal and family history, current disease, reproductive and contraceptive history was collected from cases and controls by interview. Results There were 910 cases and 2709; controls. After univariate and unconditional multiple logistic regression analysis, risk factors associated positively with uterine leiomyomas are tubal ligation, family history of uterine leiomyomas, higher education, obesity and abortion. In contrast, DMPA, use of oral contraceptives, higher parity and smoking are associated with a lower relative risk suggesting that they have a protective effect against uterine leiomyomas. This causative relation is further strengthened by the strong duration‐response relation between DMPA and uterine leiomyomas. This protection may persist for more than 10 years after the last dose. Conclusion We have demonstrated a strong, duration dependent protective effect of DMPA against uterine leiomyomas.

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