Background Methamphetamine (MA) use disorder is a new health problem among female methadone patients in Iran. Some characteristics of women are likely to be associated with poor methadone treatment outcomes. However, there are few studies about the characteristics of this group. Objectives The current study is the first research that aimed to investigate the demographic, drug use and treatment characteristics of a group of regular MA-using women in methadone treatment. Methods Methadone-maintained women who reported at least weekly MA use were recruited from four methadone treatment centres in Tehran. Participants’ drug use, severity of MA dependence, psychiatric distress, social functioning, criminality and high risk sexual behaviours were measured among the participants. Urinalysis was done to confirm MA use. Results Between 28 July 2014 and 15 May 2015, 120 participants completed an interview. Mean duration of MA use was 5.4 (SD 2.1) years. Only 9.2% of the participants reported prior MA treatment. Psychiatric distress was high among all participants. Overall, 61 participants (50.8%) reported long duration of MA use (i.e. five years or more) while 59 participants (49.2%) reported less than five years of MA use. Severity of MA dependence (P = 0.38), psychiatric distress (P = 0.36), social functioning (P = 0.12) and criminality (P = 0.82) were not significantly associated with duration of MA use. However, participants who reported long duration of MA use were more likely to be engaged in high risk sexual behaviours (P = 0.01) compared with those reporting short duration of MA use. Conclusions The study results indicated that participants had multiple health problems. High risk sexual behaviours were considerable among participants with long duration of MA use despite being in methadone treatment. Participants had little or no previous treatment for MA use disorder. Treatment services should be expanded to address MA use disorder and its harms among this group.