Illegal drugs including cocaine, heroin and cannabis should be reclassified to reflect a scientific assessment of harm, according to a report by the Global Commission on Drug Policy.

The commission, which includes 14 former heads of states from countries such as Colombia, Mexico, Portugal and New Zealand, said the international classification system underpinning drug control is “biased and inconsistent”.

A “deep-lying imbalance” between controlling substances and allowing access for medicinal purposes had caused “collateral damage”, it said. Such damage included patients in low- and middle-income countries forced to undergo surgery without anaesthetic, to go without essential medicines and to die in unnecessary pain due to lack of opioid pain relief.

Other negative consequences were the spread of infectious diseases, higher mortality and the global prison overcrowding crisis, the report said.

“The international system to classify drugs is at the core of the drug control regime – and unfortunately the core is rotten,” said Ruth Dreifuss, former president of Switzerland and chair of the commission. She called for a “critical review” of the classification system, prioritising the role of the World Health Organization (WHO) and scientific research in setting criteria based on harms and benefits.

Restrictions on milder, less harmful drugs should also be loosened, the commission said, to include “other legitimate uses”, including traditional, religious or social use.

Some illegal drugs, including cocaine, heroin, cannabis and cannabis resin, were evaluated up to 30 years ago or have never been evaluated, Dreifuss said, which seriously undermines their international control.

Asked whether these drugs should be reclassified, Juan Manuel Santos, the former president of Colombia, replied “yes”. “The scientific basis is non-existent,” Santos told journalists at an online briefing to discuss the commission’s report.

“It was a political decision. According to the studies we’ve seen over past years, substances like cannabis are less harmful than alcohol,” he said. “I come from Colombia, probably the country that has paid the highest price for the war on drugs.”

After 50 years, the war on drugs has not been won, Santos said. It had caused “more damage, more harm” to the world than a practical approach that would regulate the sale and consumption of drugs in a “good way”.

The WHO estimated in 2011 that 83% of the world’s population lived in countries with low or non-existent access to opioid pain relief.

The commission’s recent report looks into how “biased” historical classification of substances, with its emphasis on prohibition, has contributed to the world drug problem. Under the current system, in place since 1961, decisions on classifying drugs are taken by the Commission on Narcotic Drugs (CND), a body of UN member states established by the UN Economic and Social Council. The WHO Expert Committee on Drug Dependence provides recommendations to the CND. However, the recommendations are then voted on by the CND members, leaving them open to political decisions.

Helen Clark, the former prime minister of New Zealand, said the WHO should make decisions on drug classification based on health and wellbeing. More harmful drugs would require a higher level of intervention, she said.

“The international community should recognise that the system is broken,” said Clark. “They should recognise the inconsistencies and it should trigger a review.”

Risk thresholds, such as those used for alcohol, should be used for illegal drugs rather than the “absolute precautionary principle”, she said.

The commission called on the international community to move towards the legal regulation and use of drugs. In January, the WHO recognised the medical benefits of cannabis and recommended it be reclassified worldwide.

Michel Kazatchkine, French physician and former executive director of the Global Fund to Fight Aids, Tuberculosis and Malaria, said that 75-80% of the global population do not have access to medicines and “all of the reasons are linked to repression and prohibition-based control systems”.

“These restrictive policies under international control have been impeding and are continuing to impede medicines that are not only needed, but are on the WHO list of essential medicines.”

He said a “crisis of regulation” in the US had led to the “dreadful consequences” of the opioid crisis, as a result of which 72,000 people died in 2017.

“We need to think of these things with a fresh outlook,” said Anand Grover, the former special UN rapporteur for health, India. “We can’t go with the cultural biases of the west.”