After PSM, the LNG-IUD group showed a significantly lower risk of PID (hazard ratio (HR) 0.545; 95% confidence interval (CI) 0.483–0.616) and Hb <10 g/dL (HR 0.850; 95% CI 0.775–0.932) compared with the oral norethindrone group. The gene discussed is GSTM1; the disease is pelvic inflammatory disease.