Prohibition on its own terms: use, potency, markets and who paid the price

Every policy makes promises, and the fairest way to judge one is to hold it to them. Criminal prohibition of cannabis promised to protect health by reducing the number of people who use the drug, by deterring the young from starting and by suppressing supply. For most of the twentieth century those promises were the organising principle of drug law across much of the world, from the Single Convention of 1961 to the United Kingdom's Misuse of Drugs Act 1971. That gives us a long record to examine.

This article, the second in a five-part series drawing on a critical comparative review of cannabis policy, examines that record. It does not ask whether cannabis is harmless (it is not). It asks a narrower, testable question: judged on its own terms, what did prohibition achieve, and who bore its costs? The first article set out the range of legal regimes and why "legal versus illegal" is too crude a frame; here "prohibition" means the regime in which both possession and supply are criminal offences. Later articles turn to the reform models, commercial legalisation and the health evidence and regulatory tools.

Judging a policy by its own yardstick

What prohibition said it was for

The preamble to the Single Convention on Narcotic Drugs 1961, the treaty that still anchors international drug control, frames control as a duty to protect "the health and welfare of mankind" (United Nations, 1961). That is a broad aspiration. In practice, the review identifies three operational aims that national prohibition regimes pursued:

• to reduce the number of people using cannabis;
• to deter young people from starting;
• to suppress supply.

Each can be tested. A policy that promised fewer users, later starters and a smaller market can be held to account.

Why the test is harder than it looks

Policy researchers prefer designs that compare places that changed their law with similar places that did not, so that the effect of the law can be separated from changes that would have happened anyway. Prohibition spread across the world through treaties, so such clean comparisons are rare, and much of the evidence is of a weaker kind: global trends, cross-country comparisons and administrative statistics. The review also flags hazards that run through the whole field. Use was already changing for cultural and generational reasons; people report illegal behaviour less readily, so surveys under prohibition may understate use; and the product itself changed, so comparing "use" in 1975 and 2025 compares exposure to quite different substances (Freeman et al., 2021). These hazards weaken strong claims in every direction.

Did prohibition suppress use?

The global picture

On the most basic measure, no. Cannabis is now the most widely used controlled drug in the world. The United Nations Office on Drugs and Crime (UNODC) estimates 244 million past-year users in 2023 and 256 million in 2024, and reports that use grew by about a third over the preceding decade (UNODC, 2025; UNODC, 2026). "Past-year" users are people who report having used cannabis at least once in the previous twelve months; it is a wider category than regular or heavy use, but it is the standard yardstick for comparing populations.

The detail that matters most is where the growth happened: almost all of it took place in countries where cannabis remained illegal. The global increase cannot be explained away as the result of legalisation in a handful of jurisdictions. Global estimates are assembled from national surveys of varying quality and are best read as showing direction and rough scale rather than precise counts. But the direction is not in doubt, and it runs against prohibition's central promise.

Do tougher laws mean less use?

A second way to test the claim is to compare countries with different laws at the same moment. The World Health Organization's World Mental Health Surveys asked comparable questions about drug use in many countries. In that data, the United States, with some of the most punitive drug laws in the developed world, had higher rates of lifetime cannabis use than the Netherlands, which tolerated retail sales through its coffeeshops (Degenhardt et al., 2008). The authors concluded that drug use does not appear to be related in any simple way to drug policy. Prevalence appears to track cultural, economic and generational factors more closely than legal status.

A cross-national comparison is not an experiment. The United States and the Netherlands differ in countless ways besides their drug laws, so a single comparison cannot prove that strict laws have no effect. What it can do is test a strong claim: if tough prohibition were a powerful lever on use, the most punitive countries should stand out with low prevalence. They do not.

The narrower point about deterrence

None of this proves that prohibition had no deterrent effect at all, and the review is careful on this point. Some people who would otherwise have used cannabis were certainly deterred: by the risk of arrest, by the difficulty of finding a supplier, by the stigma attached to an illegal act. And the price of cannabis was probably higher than it would have been in a legal market, which matters because, for most goods, higher prices mean lower consumption.

The point is narrower. The deterrent effect was too small to deliver the policy's own goal, and it was bought at substantial cost. A policy whose aim is to keep use low cannot claim success when use has grown by about a third in a decade under its own rules, even if use might have grown somewhat faster without it.

The strongest case for prohibition

Before weighing the rest of the record, here is the best case a defender of prohibition can make. It is more serious than critics often allow.

Deterrence works at the margin. Even if prohibition did not keep use low in absolute terms, it plausibly kept it lower than it would otherwise have been. Most people obey most laws most of the time, and illegality adds friction: one has to find a seller, run a risk and accept a social stigma. The counterfactual, the world without prohibition, is not observable, so the size of this effect cannot be read straight off the data. Critics who point to rising use, defenders can fairly say, are not comparing prohibition with what would have happened under a different law.

Price matters. Illegal markets carry costs that legal ones do not: the risk of arrest and seizure, the need for secrecy, the absence of economies of scale. These costs are passed on to buyers. The review accepts that prices under prohibition were probably higher than in a legal market. Evidence from commercial markets lends this argument weight. In the United States, commercial legalisation has been followed by falling prices, rising potency and more daily use among adults (Hall and Lynskey, 2020). By 2022 an estimated 17.7 million Americans used cannabis daily or near-daily, more than the number who drank alcohol that often, although part of the long-run rise may reflect greater willingness to report use (Caulkins, 2024).

Some harms barely existed before. Accidental poisoning of young children by cannabis edibles was a harm that scarcely existed under prohibition and grew with legal edible products (Myran et al., 2022). A prohibitionist can reasonably say that at least some new harms are the product of legal markets, not merely of cannabis.

These arguments deserve weight, and later articles take them seriously. But they show that commercial legalisation brings costs of its own, and that prohibition may have held use somewhat below its unregulated level. They do not show that prohibition delivered what it promised. The review's own summary table of contested claims rates the proposition "prohibition keeps use low" as not supported (Degenhardt et al., 2008; UNODC, 2026). The defence is better understood as an argument about the dangers of the alternatives than as a vindication of prohibition's record.

Potency: the "iron law of prohibition"

A ban changes the product

One of the least anticipated consequences of prohibition was a change in what was sold. In 1986 Cowan proposed what he called an "iron law of prohibition": when a commodity is banned, suppliers shift towards more concentrated forms, because these are easier to conceal and transport per unit of value (Cowan, 1986). The logic is simple. If every shipment risks seizure and every dealer risks arrest, it pays to move as much value as possible in as small a package as possible.

What the measurements show

Cannabis followed this pattern. A meta-analysis of more than 80,000 samples from seven countries found that the THC content of herbal cannabis rose by 0.29 percentage points a year from 1970 to 2017, and that of resin by 0.57 points a year, while CBD content did not change (Freeman et al., 2021). The rise in herbal THC was driven by the growing market share of high-THC sinsemilla, the seedless flowering tops of unpollinated female plants.

A meta-analysis pools the results of many separate studies to estimate an overall trend more reliably than any one could. A rise of 0.29 percentage points a year sounds small, but it is an absolute change in the share of THC by weight, repeated for nearly half a century. THC is the main intoxicating compound in cannabis and CBD a non-intoxicating one; because THC rose while CBD stayed flat, the balance between them shifted towards THC.

Why concentration matters for health

Potency bears directly on the two most serious harms associated with cannabis. The first is psychosis. The EU-GEI study, a multicentre case-control study, recruited 901 patients with first-episode psychosis and 1,237 population controls across 11 sites in Europe and Brazil (Di Forti et al., 2019). In a case-control design, researchers start with people who have a condition (the cases) and compare their past exposures with those of similar people who do not (the controls). The result is usually expressed as an odds ratio: a figure of 1 means no association, and a figure of 3 means the odds of having the condition are three times as high among the exposed. After adjustment for other factors, daily users were about three times as likely as never-users to have a psychotic disorder, and daily users of high-potency cannabis (THC of 10% or more) about five times as likely. Assuming causality, the authors estimated that 12.2% of first-episode psychosis cases across the sites would be prevented if high-potency cannabis were unavailable, rising to 30.3% in London and 50.3% in Amsterdam.

That "assuming causality" matters: a case-control study cannot by itself prove causation, though the authors draw on wider evidence to support a causal link.

The second harm is dependence. Cannabis use disorder is the most common serious harm associated with the drug; about 42% of drug use disorder cases worldwide are attributed to cannabis (UNODC, 2025). A systematic review found that higher-potency cannabis was associated with a higher risk of cannabis use disorder than lower-potency products, with one of the strongest included studies reporting roughly four-fold higher odds (Petrilli et al., 2022). Together, these studies support the review's conclusion that high-THC, low-CBD cannabis carries greater risks of dependence and psychosis (Di Forti et al., 2019; Petrilli et al., 2022).

A trend prohibition could not touch, and legal markets have not reversed

Prohibition therefore did not merely fail to stop use. It presided over a market that delivered a more harmful product and offered the state no lever to regulate it. A government cannot cap THC content or tax by strength in a market that, legally, does not exist.

Fairness requires the other half of the picture. Legal markets have since continued and in some cases accelerated the potency trend, through concentrates and high-THC extracts. In the United States, commercial legalisation has been followed by rising potency alongside falling prices (Hall and Lynskey, 2020). In Ontario, the share of new schizophrenia cases associated with cannabis use disorder rose from 3.7% before legalisation to 10.3% after it, but the rise was steady over the whole period with no clear acceleration at the point of policy change, so it may reflect long-run increases in potency and heavy use more than legalisation as such (Myran et al., 2025). The EU-GEI data point the same way: high-potency products were widespread in London, under prohibition, as well as in Amsterdam, under tolerance (Di Forti et al., 2019).

So potency is not a problem prohibition alone created. But it is one that prohibition was structurally unable to address. A legal market can in principle cap potency or tax it; whether it does is a question of design, taken up in the final article of this series.

Illicit markets and crime

Who runs the market

Prohibition does not abolish a market for which there is demand; it transfers the entire market to unregulated suppliers. Economists have long argued that this creates violence, corruption and organised-crime revenue as by-products of enforcement, not of the drug itself (Miron and Zwiebel, 1995). Participants in an illegal trade cannot enforce contracts in court or call the police when robbed, so disputes are settled privately and sometimes violently.

The review is measured about how far this applies to cannabis. Cannabis markets are generally less violent than those for cocaine or heroin. But they still finance criminal organisations and expose buyers to dealers who sell other drugs.

No quality control

In an illegal market, buyers cannot know the THC content, the presence of contaminants or the age of the person selling to their children, and the state cannot require any of these things.

What the alternative does not automatically fix

Ending prohibition does not automatically end the illicit market. In Canada, the share of users who usually bought from a legal source rose from 37% in 2019 to 72% in 2024 (Health Canada, 2024): substantial displacement, but not complete. Uruguay's tightly restricted state model left room for a "grey market" in which legally produced cannabis is redistributed illegally (Queirolo et al., 2024), and in Germany only about 3.5% of users obtained cannabis from the new non-profit associations (EKOCAN, 2026). Prohibition guarantees that the market is criminal; the alternatives make it possible, but not certain, to shrink it, depending on design (see the third article).

Enforcement, equity and the criminal record

The heaviest and most measurable costs of prohibition were borne through the criminal justice system, and they were not borne equally. This is the part of the record where the evidence is strongest.

The United States: arrests

The American Civil Liberties Union analysed marijuana arrests in the United States from 2010 to 2018. It found that Black people were 3.64 times more likely than white people to be arrested for marijuana possession, despite similar rates of use, and that the disparity existed in every state (ACLU, 2020).

The phrase "despite similar rates of use" is the heart of the matter. Where two groups use cannabis at similar rates, a large gap in arrests points to a difference not in behaviour but in how the law is enforced.

The same report found that states that had legalised possession had, on average, lower racial disparities than states that had only decriminalised or retained prohibition (ACLU, 2020). Legalisation also sharply reduced the overall number of possession arrests. This is a comparison across states, not a controlled experiment, but it suggests that the law itself shapes the disparity.

The United Kingdom: stop and search

The United Kingdom shows the same pattern through a different police power. In England and Wales, officers may stop and search a person if they have reasonable grounds to suspect that the person is carrying, among other things, controlled drugs. Using Home Office and Ministry of Justice data, Eastwood, Shiner and Bear (2013) found that:

• Black people were stopped and searched for drugs at 6.3 times the rate of white people, despite lower reported rates of drug use;
• Black people were five times more likely to be charged for possession of cannabis;
• more than half of all stop-and-searches were for drugs.

Disproportionality has persisted: in the year to March 2024, Black people in England and Wales were about five times more likely than white people to be stopped and searched (Home Office, 2024).

In 2022 the Independent Office for Police Conduct recommended that the Home Office review what counts as reasonable grounds for suspicion of cannabis possession, including the smell of cannabis (IOPC, 2022). The review lists a reassessment of smell as grounds for stop-and-search among three UK priorities that would not require legalisation, alongside ending the criminalisation of personal possession and making medical access independent of ability to pay.

For clarity about the present law: in the UK, cultivation and possession of cannabis remain criminal offences, and growing the plant requires a Home Office licence even for patients with a prescription. This article is analytical and gives no guidance on obtaining, growing or using cannabis.

Beyond the encounter: the criminal record

The cost of these encounters goes beyond the event itself. A conviction for possession can block employment, travel and housing for years. These consequences are imposed for an activity that, on the best available evidence, harms the user more than others (Nutt, King and Phillips, 2010), and they fall more often on Black and minority citizens for the reasons set out above.

The proportionality objection

Even if prohibition did reduce use somewhat, is the criminal law a proportionate response? The best-known attempt to rank drug harms was made by the Independent Scientific Committee on Drugs using multicriteria decision analysis. This method breaks a complex judgement into many separate criteria, scores each one, weights them by importance and combines them into an overall score. Here an expert panel scored 20 drugs on 16 criteria of harm, some to users and some to others. Alcohol scored highest overall (72 out of 100), heroin 55 and crack cocaine 54, while cannabis scored 20 (Nutt, King and Phillips, 2010). The authors noted that their ranking correlated poorly with the UK classification system.

The method has real limitations. Its results depend on the judgement and weighting of a small expert panel, and it scores harms as they occurred under existing laws, which mixes up the harms of a drug with the harms of its policy. Even so, its central finding, that the legal status of drugs bears little relation to their relative harm, has not been seriously overturned. Proportionality is therefore a central ethical objection to criminal prohibition, independent of its effect on prevalence.

The UK's own history illustrates how loosely classification has tracked evidence. Cannabis was moved from Class B to Class C in January 2004 and back to Class B in January 2009, the second change made against the advice of the Advisory Council on the Misuse of Drugs (Advisory Council on the Misuse of Drugs, 2008). The review reads that reversal as a sign of how far classification decisions have been political rather than scientific (Nutt, King and Phillips, 2010).

A legitimacy problem: moral panic and the making of prohibition

Why has prohibition's legitimacy been so persistently contested, even by people who accept that cannabis carries real risks? Part of the answer lies in how the laws were made. The sociologist Cohen described a moral panic as a process in which a group is cast as a threat to social values and the reaction becomes disproportionate to the danger (Cohen, 1972). The review argues that the concept fits much of the history of cannabis control. The US campaign that produced the Marihuana Tax Act of 1937 drew heavily on racialised imagery of Mexican migrants and Black musicians. British support for the 1925 Geneva controls owed more to imperial diplomacy than to evidence of harm (Mills, 2003). And the official inquiries of the period, from the Indian Hemp Drugs Commission of 1894 to the Wootton Report of 1968, repeatedly reached more moderate conclusions than the laws that followed (Mills, 2003; Advisory Committee on Drug Dependence, 1968). That history is told in more detail in an earlier article on medicine and the making of prohibition.

This should not be overstated. A law with disreputable origins is not thereby a bad law, and the review says plainly that these origins do not by themselves show that prohibition was wrong. But they help explain why its legitimacy has been contested for so long, and why its enforcement has fallen so unevenly.

Barriers to medical research

Prohibition also shaped what we know about cannabis. In the United States, cannabis was placed in Schedule I of federal drug law, the category reserved for substances deemed to have a high potential for abuse and no accepted medical use. The US National Academies of Sciences, Engineering, and Medicine concluded that regulatory barriers, including that classification, had impeded research on the health effects of cannabis, both harmful and therapeutic (National Academies of Sciences, Engineering, and Medicine, 2017).

The consequence is a kind of circularity. Prohibition reduced the evidence available to regulators and patients, which in turn made it harder to judge the policy itself. It slowed research into risks as well as benefits; when the evidence was eventually assembled, the National Academies found conclusive or substantial evidence of benefit for only a few indications, such as chronic pain in adults.

Weighing the record

How does prohibition fare against the three aims set out at the start, and against the wider costs?

The review's interim assessment is plain. Judged against its own aims, prohibition performed poorly. It did not prevent cannabis from becoming the world's most widely used illicit drug. It coincided with steadily rising potency it could not regulate. It handed the market to criminal suppliers and imposed costs on users that fell disproportionately on ethnic minorities. Its one plausible success, keeping prices higher and use lower than they would otherwise have been, is real but modest and hard to measure.

Both sides overstate here. The evidence does not show that prohibition had no effect on use, nor that any effect justifies the cost. The honest position is that prohibition probably held use somewhat below its unregulated level, at a price paid largely by people who were arrested, searched and given criminal records, disproportionately from minority communities.

The case against prohibition is therefore strong. Whether the alternatives perform better is a separate question, taken up in the next article and in the one on commercial legalisation.

What we know, and what we don't

What the evidence supports well:

• Global cannabis use grew substantially under prohibition, and cross-national comparisons do not show that tougher laws produce lower use (UNODC, 2026; Degenhardt et al., 2008).
• The THC content of illicit cannabis rose steadily from 1970 to 2017 while CBD did not, consistent with the "iron law of prohibition" (Freeman et al., 2021; Cowan, 1986).
• Enforcement of cannabis laws in the United States and the United Kingdom has fallen disproportionately on Black people, despite similar or lower reported rates of use (ACLU, 2020; Eastwood, Shiner and Bear, 2013; Home Office, 2024).
• Regulatory barriers impeded research on both the harms and the benefits of cannabis (National Academies of Sciences, Engineering, and Medicine, 2017).

Where uncertainty remains:

• The size of prohibition's marginal deterrent effect, and of its effect on price, cannot be measured directly, because the counterfactual world without prohibition is not observable.
• Survey estimates of use are sensitive to stigma, so trends under prohibition and after reform may partly reflect changes in willingness to report.
• The link between high-potency cannabis and psychosis is robust as an association; the estimates of preventable cases rest on the assumption that it is causal (Di Forti et al., 2019).
• Potency has continued to rise in some legal markets, so the potency problem cannot be attributed to prohibition alone (Hall and Lynskey, 2020; Myran et al., 2025).
• The multicriteria ranking of drug harms depends on expert judgement and on harms as they occurred under existing laws (Nutt, King and Phillips, 2010).

This article is based on the critical review it expands; figures are reported as that review gives them and were not re-checked against every original study.

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