So often when people think about drugs and those who use them, they conjure up images of “junkies” injecting heroin in the street or young professionals “throwing away” their promising careers because they are “hooked” on cocaine. At the same time, alcohol is seen more as a social lubricant than a substance capable of inflicting significant harm, and tobacco users are not seen in pejorative terms despite the deadly drug they are consuming.

Science is clear, however, that alcohol and tobacco are psychoactive substances with addictive and harmful elements. What separates them in our minds is their legal status, which itself is defined by a flawed assessment process that purports to classify substances according to their potential for harm. In truth this system is broken almost beyond repair.

Take two well-known illegal psychoactive substances – cannabis and heroin.

There are no known lethal overdoses of cannabis. The risk of dependence on it is also relatively low, with an estimated nine per cent of those who consume it exhibiting problematic use. Heroin, on the other hand, is deadly when consumed in high doses – which can happen quite easily when street heroin of unknown quality is “cut” with powerful synthetic adulterants or fentanyl, and its use leads to dependence for about 23 per cent of those who try it.

This mismatch between science and international obligations and national policy leads to absurd – and often tragic – situations.

Teenager Billy Caldwell from the UK was suffering from life-threatening epileptic seizures several times a day until he received cannabis oil in the US which stabilised his condition. Yet what was a medicine in the US was confiscated by authorities in the UK as an illegal drug. In this case, the absurdity became clear enough for the UK to introduce “Billy’s law” granting limited access to medical cannabis.

At this very moment, however, Billy is having to fight yet again to get a prescription through his local health services instead of through costly private consultations. If they lose this battle, his family is considering moving to a country where the “drug” that their son needs is considered a “medicine”.

The truth is that whether a substance is legal or illegal, and whether it is regarded as a “drug” or a medicine, has more to do with history, geography and culture than with its pharmacological qualities or the harms it may cause.

Alcohol and tobacco, for example, are much deadlier substances than we usually acknowledge. In 2012, 3.3m deaths worldwide were linked to alcohol, while tobacco eventually kills about half of its consumers. They were, however, the socially accepted drugs in the home countries of the main negotiators of the1961 Convention, so placing them under international control was never considered.

By contrast, the traditional use of the coca leaf in the Andean region as a mild stimulant became prohibited until one country, Bolivia, fought back. It withdrew from the UN drug treaty in 2012 and rejoined a year later, claiming an exception that allowed the customary chewing of the leaves to continue.

Another example of cultural bias is the Psychoactive Substances Act 2016in the UK. Certain substances such as caffeine, nicotine, alcohol and tobacco were exempted from the ban as “legitimate” substances without any scientific evidence as to their relative harms compared with other currently illegal drugs.

This process of classification is also subject to political considerations. Until 1961, the decision on classifying psychoactive substances was in the hands of specialised health agencies and their experts. Now, however, experts only provide recommendations, which are then voted on by the Commission on Narcotic Drugs, a body of representatives of UN member states.

Ideological considerations – such as a desire to “eradicate drugs” in order to protect society because drug use is seen as morally “bad” and dependence as an “evil” – are therefore able to trump scientific assessments of risk.

There is a need for fundamental reform to the way in which drugs are evaluated and by whom. In the new report of the Global Commission on Drug Policy Classification of Psychoactive Substances released today, we propose the guiding principles for a science-based system as the foundation for rational drug policy.

A multi-disciplinary, scientifically solid, cost-benefit analysis of potential harms and perceived benefits has to be carried out, beginning with the most used substances.

We argue for definitions of what risk threshold is socially acceptable and for authorities to communicate clearly on this. The high numbers of deaths from alcohol and tobacco are well known, but the public is generally not aware, for instance, that more people die from paracetamol overdoses than from amphetamines.

All legally available medicines have risks and side effects which doctors, authorities, and, ultimately, societies have deemed acceptable on balance with their therapeutic benefits. We need to weigh those differently for non-medical uses, but the acceptable level is certainly not zero – otherwise alcohol, tobacco and caffeine would have been banned long ago.

We suggest that we develop policies that incentivize people away from the more harmful substances by using medical and consumer safety regulations and other measures outside the criminal justice system. We need a public health approach to all problematic use of psychoactive substances.

Helen Clark is a former Prime Minister of New Zealand and former Administrator of the United Nations Development Programme.

Ricardo Lagos is a former President of Chile; members of the Global Commission on Drug Policy.