Objective To study the association between knowledge about levonorgestrel emergency contraception (LNG-EC) and the risk of ectopic pregnancy (EP) following LNG-EC failure. Methods This study included 600 women who had visited the hospital with LNG-EC failure. Of these, 300 with EP and 300 with intrauterine pregnancy (IUP) were recruited to the EP group and IUP group respectively. The participants were interviewed face-to-face using a standardized questionnaire. Main Outcome Measures Pearson's chi-square tests and t-test were used to compare the sociodemographic characteristics, reproductive and gynecological history, surgical history, previous contraceptive experience, and answers to 10 questions concerning the knowledge about LNG-EC. Results Those who gave incorrect answers to the question regarding the basic mechanism and specific method of levonorgestrel emergency contraceptive pills (LNG-ECPs) were at a higher risk of EP after LNG-EC failure. Women who did not strictly follow instructions or advice from healthcare professionals were more likely to subsequently experience EP (p < 10(-4)). Women with LNG-EC failure reported friends/peers, TV, and Internet as the main sources of information. No difference was observed with regard to the sources of knowledge on LNG-EC (p = 0.07). Conclusions The results illustrate the importance of strictly following the doctor's guidance or drug instructions when using LNG-ECPs. The media should be used to disseminate information about responsible EC, and pharmacy staff should receive regular educational training sessions in this regard. (C) 2016 The Authors. Pharmacoepidemiology and Drug Safety published by John Wiley & Sons Ltd.