The media often present cannabis policy as a simple choice that depends on how it affects users’ health. According to this view, if cannabis is harmful to users, especially adolescents, its use should be prohibited; and if cannabis is less harmful than alcohol, adults should be free to use it in the same way as alcohol [1]. The first view has long dominated cannabis policy debates, but the latter view is now in the ascendant, with the legalisation of recreational cannabis use in Uruguay in 2013, eighteen US states since 2012, Canada in 2018, and Thailand in 2021. Germany, Luxembourg, Mexico, and Switzerland are among the countries proposing to follow their example.

Supporters of the former view have often overstated the harms of cannabis by assuming that any associations between cannabis use and adverse health outcomes are causal. They have also ignored the harms caused by criminal penalties for personal cannabis use that have often fallen more heavily on socially disadvantaged minorities, such as people of colour in the USA. Advocates of more liberal cannabis policies, by contrast, arguably overemphasise the harms of prohibition, minimise the health risks of cannabis use and exaggerate the medical benefits of cannabis use.

Does cannabis use harm the health of users?

The clearest adverse effects of cannabis are an increased risk of motor vehicle crashes, if users drive while stoned. Cannabis users can experience unpleasant psychological experiences of anxiety and paranoia, if they have had limited experience with the drug, or they receive much larger than intended doses of THC.

Daily cannabis use can harm users [2], especially when use begins in adolescence and continues throughout young adulthood. This pattern of use produces cannabis dependence [3], a disorder in which users find it difficult to control or cease using cannabis, despite acknowledging that their use may be harming them or others. Regular cannabis use and dependence are, in turn, associated with higher risks of developing psychosis, having poor mental health and under-achieving in school. Troubled and socially disadvantaged young people – most often young men – most likely to engage in this pattern of cannabis use, spending most days intoxicated, much like heavy drinkers do.

The health effects of daily cannabis use are probably not as serious as the effects of daily heavy drinking because cannabis does not cause liver disease and cancers or impair brain function as severely as alcohol [4]. This is not a ringing endorsement of cannabis use, given how much harm heavy drinking causes to drinkers and their families and communities. Young people who spend most of their adolescence and young adulthood ‘stoned’ are more likely to leave school early and have more limited choices of work after school [5]. They are also less likely to form satisfying personal relationships or friendships except with other heavy cannabis users.

Cannabis policy requires unavoidable trade offs

Formulating cannabis policy unavoidably involves making trade-offs between the costs and benefits of prohibition, on the one hand, and the costs and benefits of a legal cannabis market on the other.

Prohibition does not eliminate cannabis use, as its critics argue, but it does reduce the prevalence of regular use. For example, in 2019 the percentage of Australians [6] who use cannabis in their lifetime (36 per cent), is less than half that of alcohol (90%). The percentage who used cannabis in the past year (11.6 per cent) was also a fraction of that who have used alcohol (86%). Fewer than 4% of adults used cannabis in the last week whereas 35% of adults drank alcohol and 14% smoked cigarettes.

Any benefits of prohibition come with social and economic costs [7]. The most obvious is the cost of enforcing the criminal laws against cannabis producers, sellers, and users. As critics point out, these laws disproportionately affect socially disadvantaged males from minority groups, who predominate among regular cannabis users [8].

Legalisation of cannabis products and sales, by contrast, clearly benefits cannabis users. It eliminates criminal penalties for use and provides them with easier access to regulated cannabis products at a reasonable price. Some criminal laws still need to be enforced – for instance, laws prohibiting under-age sales and the illicit production and sale of cannabis products. Governments also need to tax and regulate the legal cannabis industry [9].

Tax revenue from cannabis sales can fund cannabis regulation and treatment and prevention programs, like gambling taxes are claimed to do. Cannabis taxes must not be too high, however, if a key policy aim is to reduce the size of the illicit cannabis market.

Tax revenue from legal cannabis sales may also become a policy trap for governments. Like tax revenue from gambling, revenue from cannabis comes disproportionately from the heaviest users, especially daily users [10], often the most socially disadvantaged users. Governments may become hostage to the cannabis industry, just as the Australian government has become to the “gaming” industry, which has to date prevented any restrictions on the size of gaming bets or losses and facilitated the use of gaming machines for money laundering.

The legalisation of cannabis production and retail sales creates a new industry with an interest in expanding cannabis sales and their profits. Cannabis retailers will have an interest in promoting regular cannabis use because these users consume most of their product and so generate their greatest profits. They will also have an interest in increasing the number of new cannabis users via sales promotions and campaigns to “destigmatise” and normalise cannabis use while discounting any evidence that it can harm users. A profitable and well organised cannabis industry – like the alcohol, tobacco, and gambling industries – will prove adept at minimising taxes and regulations that they will argue reduce their capacity to compete with the illicit cannabis market.

How has cannabis legalisation worked out in those states in the USA where cannabis production, sales and use has been legal for adults since 2012?

2. How did adult cannabis legalisation in the USA come about?

For much of the past 40 years, the US has had the highest rates of cannabis use and the most punitive policies towards cannabis users in the developed world. Personal cannabis use was briefly decriminalised in half the US states during the 1970s when rates of use among youth were at their highest but punitive policies were renewed during the Presidency of Ronald Reagan.

Since 1996 some form of cannabis use for “medical purposes” has been legalised in 38 US states. This initially occurred by citizen-initiated referenda when a proposal that attracted a required number of citizens’ signatures (e.g. 100,000) was put to the vote. If the measure was passed by a majority, a law to implement the policy had to be enacted by the state legislature. US states later legislated to legalise medical cannabis use.

Liberal medical marijuana laws arguably paved the way for the legalisation of recreational cannabis use. Advocates defined medical use very broadly, allowing large numbers of users to obtain an indemnity against arrest under state law. Some states also allowed commercial “dispensaries” to supply cannabis to “patients” who had a doctor’s recommendation. This created a de facto legal cannabis market in states like California where 10 per cent of males aged 18-24 years reported “medical use”. It also increased public support for adult legalization [11] between 2009 and 2013 and thereby enabled the passage of referenda to legalise cannabis in Colorado and Washington State in 2012 and Alaska and Oregon in 2014. Similar proposals were supported in 2016 and subsequent years and US states including Illinois, New Jersey, New York and California legalised adult cannabis use by passing legislation.

How have legal cannabis markets been regulated?

The first US states to legalise cannabis, Colorado, Oregon and Washington State, decided to regulate cannabis in much the same way as they regulated alcohol. They limited use to adults over the age of 21 (the legal drinking age), licensed cannabis producers, processors and retailers, and imposed a state tax on cannabis sales. As promised, cannabis legalisation has allowed adults to use cannabis without fear of arrest, provided these states with a new source of tax revenue, and reduced the costs of enforcing the criminal law against cannabis users.

A major challenge for US state governments in regulating legal cannabis markets is that it is still a criminal offence under US Federal law to produce, use and sell cannabis [12]. Under the Constitution, Federal laws pre-empt state laws when the two conflict, as the US Supreme Court ruled in 2001. The Department of Justice could take legal action in the US Supreme Court to over-ride the state laws, it could prosecute producers, sellers and state officials for breaking Federal laws or it could confiscate the profits of legal cannabis sellers as the proceeds of criminal activity under Federal law.

Since 2008 no US Federal government has adopted any of these options. In 2013 the Obama administration announced that it would give a low priority to enforcing Federal law in states that legalised retail sales of cannabis, provided these states regulated cannabis in ways that did not endanger public health or public order. The Department of Justice reserved the right to enforce Federal law if these priorities were not respected. This policy did not change after the election of Donald Trump in 2016. Republicans were less supportive of cannabis legalisation than Democrats, but they also preferred to leave these issues to state governments.

18 US states have now legalised cannabis, including the largest, California. These states include nearly half of the US population. As more states have legalised, the Federal government has come under increased pressure to remove the Federal prohibition on cannabis by, for example, removing cannabis from the Federal Controlled Substances Act (CSA). Despite these legal constraints the promotion of cannabis has increased in the USA. Glossy magazines glamorise cannabis products and use, celebrities endorse cannabis strains.

The potency of cannabis products (measured by THC content) has substantially increased since legalisation. The THC content of herbal cannabis is now around 20% (compared to 10% before legalisation) and sales have increased of cannabis extracts that contain 70% or more THC. There is now a growing trade in ‘fortified’ pre-rolled joints: pre-rolled herbal cannabis joints to with THC has been added to increase their THC content to 30%. Cannabis is now far cheaper and more readily available to users than was the case under cannabis prohibition. [13] Given our experience with the effects of reducing the price and increasing access to alcohol, one would expect these changes to increase cannabis use [14].

3. Has legalisation affected cannabis use and cannabis-related harms in the USA?

Researchers studying the impacts of legalisation on cannabis use in the USA have compared household survey data on rates of cannabis use in US states that allow medical or adult cannabis use with states that do not [15]. These studies have produced consistent evidence that the prevalence of regular cannabis use has increased among adults in US states that have legalised adult cannabis use. They have not so far found consistent increases in adolescent cannabis use but it is of concern that adolescents perceive cannabis use to be less risky than they used to do because in the past these attitudinal changes have predicted subsequent increases in adolescent use [16].

The concern that legalisation will increase adolescent cannabis use is understandable. Daily cannabis use in adolescents has been strongly associated with: educational underachievement; poorer mental health; and the use of other illicit drugs. It is difficult to tease out cause and effect because troubled young people are more likely to become regular cannabis users. It is nonetheless likely that regular cannabis use by troubled young people will not improve their well-being and quality of life. The public health challenge will be finding effective ways to prevent regular use among adolescents when adult use is legal.

A major public health concern is that cannabis legalisation will increase car crashes deaths and injuries if more cannabis users drive while impaired. There is debate about the strength of evidence that cannabis impairs driving; studies of the effects of legalisation on impaired driving are also conflicting. All US states that have so far legalised have nonetheless banned driving while impaired by cannabis; the challenge has been in enforcing these laws.

As noted above, cannabis is a drug of dependence in that cannabis users can find it difficult to control their use or cease using, despite their use causing harm to themselves and their family members. As the prevalence of cannabis use increased in many high-income countries, more persons who use cannabis have sought help from addiction treatment services. This has reflected in part treatment under legal coercion (referral by the courts) but this is not the whole explanation because similar increases occurred in the Netherlands where cannabis use was decriminalised in the 1970s [17]. The risks of developing dependence on cannabis were lower than those for alcohol, tobacco or heroin in the early 1990s but the risk may have increased because more potent cannabis products (e.g. concentrates with 70 percent THC) are now sold at lower prices in US states that have legalised [18].

As noted above, adult cannabis legalisation has increased the prevalence of daily cannabis use so one would expect to see an increase the prevalence of cannabis dependence but research on the issue has been mixed. The number of persons seeking treatment for cannabis problems has declined because fewer persons are entering treatment under legal coercion and two major US household surveys of patterns of cannabis use have produced conflicting findings: one found an increase in the prevalence of cannabis dependence; the other did not [19].

The numbers of persons presenting to emergency departments (ED) reporting adverse psychological effects of cannabis use have increased in US states and in Canada. The acute adverse psychological effects include anxiety and paranoia that are most common in persons who have used edibles cannabis products and received much higher doses of THC than intended. ED attendances have also increased among longer-term heavier users such as cannabis intoxication, psychotic symptoms, and a hyperemesis syndrome [20].

Why it may be too early to assess the full effects of US cannabis legalization

Cannabis legalisation in the USA has so far had only modest adverse effects. There are several reasons why it would be premature to assume that these will the longer term effects of cannabis legalisation on public health and social well-being.

The main reason is that cannabis use remains illegal under US Federal law. This has substantially hampered the commercialisation of the legal cannabis industry. Fear of Federal prosecution has limited investment in cannabis by the alcohol, tobacco, beverage and finance industries. Banks have been reluctant to deal with cannabis businesses and the Internal Revenue Service still does not allow cannabis businesses to deduct the cost of doing business from their taxable income, thereby imposing a substantial de facto Federal tax on cannabis.

The cannabis industry is vigorously lobbying for cannabis to be removed from the Controlled Substances Act. When this occurs, the cannabis industry will be allowed under the US constitution to advertise and promote the sale of cannabis and to produce and sell cannabis nationally. Cannabis prices per gram of THC have already declined by 50% since legalisation but we can expect much larger reductions in prices if legalisation becomes national US policy. And these changes are likely to increase the prevalence of regular cannabis use.