Methadone has a long history of pain relief and successful substitute for maintenance treatment in heroin and narcotic addiction. The aim of the study was to assess the trends of methadone-associated deaths in Tehran, Iran, in 2009-2015, from a forensic toxicology point of view. All methadone-associated deaths during this 7-year study period were evaluated according to demographic parameters and forensic toxicology analysis results. Results showed that 1274 cases of methadone-associated deaths were investigated during the study period. The incidence rate of methadone-associated deaths had risen 7.7 times in 2015 in comparison with 2009 (p < 0.05). The majority of cases were men (90.35%), aged from 20 to 40 years. About 80% of cases had shown positive results for other drugs and poisons in combination with methadone. Methamphetamine and tramadol were the most drugs detected in post-mortem samples. Death rates among methadone users in Tehran, Iran, increased year by year during 2009-2015. These findings raise the attention to the concomitant use of drugs with the need for changes in regulation and regulatory policy to restrict access and use of controlled drugs.