The letter, sent to UNODC Chief Executive Yury Fedotov last week, stated that some 318 people have been executed for drug trafficking in 2014 alone.
The rights defenders called for a "temporary freeze or withdrawal of support", which, as the groups pointed out, is stipulated in the UNODC human rights guidance in case a country continues executions for drug-related crimes, despite high-level political intervention.
Iran's anti-drugs laws presuppose a death sentence for manufacturing, trafficking, possession or trade of large quantities of certain drugs. According to HRW, capital punishment for drug-related offenses is not appropriate. Despite recent calls for reform by the chair of Iran's Human Rights Council, Mohammad Javad Larijani, executions of those charged with drug-related crimes continue.