Mammographic breast density (MBD) value is currently one of the strong predictors for mammary carcinoma development. There are also other conditions predisposing to MBD increase with hormone-related markers different from those used in breast cancer, while pharmacological methods for MBD reduction are few and still considered experimental. In the current study 25 postmenopausal women received daily for a median 10.5 months 1-1.5 g of antidiabetic biguanide metformin (siofor) (n = 14) or 400-600 mg of antigenotoxic drug N-acetylcysteine (n = 11). In both groups MBD was measured before and after treatment. The effects of both drugs were quite similar. Metformin use lead to lower MBD in 4 of 14 (28.5%) women with mean MBD decrease of -1,24% (absolute dynamics) and -5.03% (relative value). In N-acetylcysteine group this effect was observed in 27.3% of cases, with -2.0% absolute dynamics and -6.1% relative dynamics. In metformin group the most evident absolute and relative dynamics was observed in patients with no signs of metabolic syndrome, -10.86% compared to -2.45%. In 7 women the metformin use also lead to decrease of dense and increase of non-dense areas on digital scans, leading to decrease in dense to non-dense area volume ratio. Therefore, the similar effects of metformin and N-acetylcysteine are probably explained mostly not by insulin resistance elimination by metformin, but by altered cell proliferation, apoptosis and DNA repair.