A significant number of patients treated for heroin addiction suffer from sexual disorders. The aim of this research was to investigate the sexual function of the heroin addicted patients taking a methadone maintenance treatment (MMT), to identify the types of sexual disorders and analyze their causes. Methods: Questionnaire, scale ASEX and WHO-ASSIST V3.0 were performed by 20 sexually active patients (15 men and 5 women), undergoing MMT. Results: The data of the questionnaire have shown that 55% of the patients suffer from deterioration of their sexual function, 30% of the patients have observed the improvement of their sexual function, and 15% of the patients have not felt any changes. The patients with a worsened sexual status have taken higher daily dosage of methadone (66, 8 ± 24,11 mg). 40% of the patients have expressed the anxiety caused by the disturbance of their normal sexual function, it being known that the duration of their treatment was longer and the dosage of methadone was higher (p ˃ 0,05). According to ASEX scale, the men scored 14,13 ± 2,85 Likert points, and the women - 15,6 ± 2,88 Likert points; 5 patients (25%) have not had any sexual disorders, 4 patients(70%) have observed some disorders of their sexual functioning, and only one patient (5%) has suffered from sexual dysfunction. In general, a fade of the sexual desire (3, 25 ± 1,06 Likert points) and problems with achieving orgasm (3,0 ± 0,85 Likert points) were noticed, but the experienced orgasm was satisfactory (2,6 ± 0,99 Likert points). The patients showing relatively good indicators for the libido (2.5 Likert points), the arousal (2.3 Likert points), the satisfaction with orgasm (2,0 Likert points) took 40-50 mg of methadone per day. A marked decrease of the sexual desire, the disturbance of the ejaculatory function (3.5 Likert points), the dissatisfaction with orgasm were noted at the dosage of 80-100 mg of methadone per day. Conclusions: The methadone has an effect on the sexual function. High doses suppress the sexual desire, delay the ejaculation and prevent the achievement of orgasm, the average doses do not lead to obvious sexual disorders caused by the abuse of heroin.