Background: It has been shown previously that novel formulation of 3,3'-diindolylmethane (DIM) substance with high bioavailability (Infemin) inhibits tumor development due to the tumor growth rate reduction in the xenograft model of prostate cancer. Prostatic intraepithelial neoplasia (PIN) is considered to be promising as a personalized and preventive treatment strategy of prostate cancer (PC). We assessed the safety of Infemin in men with PIN and discussed the interim results.Materials and methods: A total of 14 patients with PIN were enrolled. They were randomized to 900 mg DIM or placebo daily for 3 months. Safety was evaluated by adverse events (AEs), laboratory tests and physical examinations.Results and conclusion: The trial revealed that Infemin treatment is associated with minimal toxicity and no serious adverse events when administered orally for 3 months. We noted three adverse events including nausea and diarrhea in two patients (14%). Combined 95% confidence interval (CI) was 1.8%-42.8%. Therapy was continued in all cases of adverse events.Good tolerability of DIM-based formulation allows us to recommend it for further clinical trials among men diagnosed with PIN for its efficacy and long-term safety parameters. © 2014 Paltsev et al.