Global efforts to abolish the death penalty are in danger of being undermined by anti-drug governments that use capital punishment to enforce a zero-tolerance approach, experts have warned.

The caution comes even though the number of people sentenced to death for drug offences around the world has actually fallen by nearly 90% over the past four years, according to a study by Harm Reduction International, with 91 known deaths last year compared with 755 in 2015.

Giada Girelli, HRI’s human rights analyst and the report’s author, welcomed the decrease in deaths but warned people are still being killed for minor drug offences.

“The fall in executions is undeniably positive, but far too many people are still sentenced to death row, where they suffer serious human rights violations, for low-level drug offences,” said Girelli. “There is simply no evidence that the death penalty serves as a deterrent, and this inhumane practice must be abolished immediately.”

With populist anti-drug rhetoric resurgent around the world, the study raises fears that global progress on abolishing the death penalty could be reversed.

Bangladesh has instituted a brutal anti-narcotics crackdown, with the prime minister, Sheikh Hasina, ordering police to deal with the drugs trade using tactics similar to those they would employ to counter violent extremism. In Sri Lanka, where the death penalty has not been used since 1976, President Maithripala Sirisena – who has praised the brutal campaign against drugs in the Philippines as an “example to the world” – recently announced that convicted drug dealers will be hanged.

Donald Trump has talked up the prospect of capital punishment for drug traffickers in the US, while the Singaporean government, having previously pursued a zero-tolerance policy that earned the US president’s approval, is discussing whether to abolish the death penalty following an international outcry over disproportionate punishments. All executions carried out in the island state last year were for non-violent drug offences.

Although drug offences can still be punished by death in at least 35 countries and territories, only four states carried out such sentences last year. At least 59 people were killed in Saudi Arabia, while a minimum of 23 died in Iran and nine in Singapore.

Death sentences were also imposed in China, but the country’s figures are shrouded in secrecy, said Girelli, who warned the actual number of deaths in 2018 was probably far higher than the 91 noted in the study.

The sharp global reduction in death sentences was largely driven by landmark reforms in Iran, where the threshold for capital punishment owing to drugs possession was raised amid a growing sense that mandatory sentences fail to deter drug use and trafficking. “The truth is, the execution of drug smugglers has had no deterrent effect,” said Mohammad Baqer Olfat, the deputy head of Iran’s judiciary. The number of people put to death in Iran following criminal justice proceedings went from 725 a year in 2010, to 221 in 2017.

Globally, at least 7,000 people – many illiterate and from impoverished backgrounds – are on death row after being forced to act as “drug mules”, according to the study. “In short, it appears that the death penalty for drug offences is primarily reserved for the most marginalised in society,” wrote Girelli.

Experts say the death penalty fails to impact the drug trade, inflicting misery on poverty-stricken families while failing to ensnare key players because they can afford expensive legal defences.

“Those sentenced to death for drug offences are often people at the lowest level of the trade, a number of whom may have entered it out of coercion or simply having no economic choice,” Professor Adeeba Kamarulzaman, dean of medicine at the University of Malaya, said. “In these scenarios, the legal system will only exploit their indigence, as stories of no access to legal aid and sham trials are all too common.”