Intervention program
The participants allocated to the intervention group were visited by a physician every two weeks, and then received a two-week supplement of zinc combined with methadone as planned. The daily dosage of the zinc supplement was about 12 mg. The supplement used in this study was Suzin Zinc Sulfate, made by Alhawi Pharmaceutical Co., Iran (of note, each capsule of 110 mg of zinc sulfate was equivalent to 25 mg of zinc). To check the participants’ cooperation in the intervention group, as well as their regular zinc supplement intake, their families were contacted three times a week. The researcher also welcomed their questions and ambiguities.On the other hand, the participants in the control group only received methadone, according to the treatment plan, and their visits were fixed every two weeks to check their methadone use by a physician or a nurse. Of note, no placebo was given to this group. All the study participants were also asked not to change their diet and type of medication at the end of the study. The methadone dose was registered in both intervention and control groups.