Background: The article presents the results of a clinical trial on the efficacy and safety of a novel pharmaceutical composition in the form of vaginal suppositories containing diindolylmethane in the course of cervical intraepithelial neoplasia (CIN) I-II conservative treatment. It offers an attractive drug therapy for more personalized prevention of cervical cancer. Methods: A total of 78 women of reproductive age were included. This was a multicenter, randomized, placebocontrolled, double-blind, parallel-group trial with efficacy determined by histological evaluation of cervical biopsies. The efficacy of active drug treatment (100 and 200 mg/day) in both treatment groups was significantly higher in comparison with the placebo group, according to the primary efficacy end point (proportion of patients with complete CIN regression after 90-180 days of the study drug treatment). Results: The efficacies were 100.0 % (confidence interval (CI) 95 %: 82.35-100.00 %), 90.5 % (CI 95 %: 69.62-98.83 %), and 61.1 % (CI 95 %: 35.75-82.70 %), for the high dose, low does, and placebo, respectively. Adverse events in the placebo group were reported in 22 % of patients (CI 95 %: 7.5-43.7 %); in the first treatment group (100 mg/day), adverse events were reported in 40.0 % of patients (CI 95 %: 21.1-61.3 %); in the second treatment group (200 mg/day), adverse events were reported in 42.0 % of patients (CI 95 %: 22.1-63.4 %). The differences in side effects between treatment groups treated with the active drug and placebo were statistically significant. No serious adverse events were reported in any of the groups. Conclusions: Thus, the use of diindolylmethane in the form of intravaginal suppositories can be effective in patients with CIN I-II and is not accompanied by clinically significant side effects. This approach could be a better option for young women with CIN I-II as it takes in attention their reproductive plans. © 2015 Ashrafian et al.