Teenage cannabis problems rose after legalisation, a study of 637,000 young people finds
What happened
A new study has looked at what happened to teenagers in one part of the United States after cannabis was made legal for adults. The research was published on 15 September 2026 in the journal Addiction. It was led by Alisa A. Padon and colleagues at Kaiser Permanente and the Public Health Institute in California. The team looked at the health records of about 637,000 young people aged 12 to 17 in Northern California. They did not ask teenagers to fill in a survey. Instead they counted how many were actually diagnosed by a doctor with cannabis use disorder. That is the medical term for when cannabis use starts to cause real harm to someone's life and they find it hard to stop. California voted to legalise cannabis for adult recreational use in 2016. Shops selling it opened in 2018. The researchers tracked diagnoses before and after those dates. In the three years before the vote, diagnoses were falling. After the law passed, they started to rise again, even before a single shop had opened. They kept rising until early 2020, when the Covid-19 pandemic began and daily life changed for everyone. The researchers say this matters because most reports on cannabis and young people rely on surveys that ask teenagers how often they use it. Those surveys had suggested that use was steady or falling. The medical records told a different story.
Why this matters
This is a study from the United States, not the United Kingdom. Cannabis remains a Class B controlled drug here, and recreational sale is not legal. So nothing in this study changes the law in Lincolnshire or anywhere else in the UK. But it still matters, for two reasons. First, the UK has an ongoing public debate about how cannabis should be treated. Careful evidence from places that have already changed their laws is worth paying attention to, whatever your view. Second, and more usefully for families, the study says something about measurement. If you only ask teenagers how often they use something, you may miss the smaller group whose use has become a real problem. Counting how many use a drug and counting how many are harmed by it are two different things. That is a point many parents and teachers will recognise. A class survey might show that not many pupils use cannabis regularly. That tells you very little about the one or two who are struggling.
What the evidence actually says
It is worth being precise about what this study can and cannot show. The design is called an interrupted time series. Researchers track a measurement over many years, then look for a change in the trend at the point when something happened, in this case a change in the law. It is a well-respected method, and 637,000 young people is a very large group. But it is still an observational study. It cannot prove that legalisation caused the rise. Other things were changing at the same time. Cannabis products themselves have become much stronger over the past decade. Attitudes shifted. Doctors may also have become more willing to record a diagnosis once the drug was legal and easier to talk about openly. The researchers are honest about this. Their main conclusion is not 'legalisation causes addiction'. It is narrower and more practical: when countries or states judge whether a change in cannabis law has worked, they should track diagnosed disorders alongside the usual surveys of how many people use it. Otherwise they may miss the harm. One more thing is worth noting. The rise began after the vote but before shops opened. That suggests the change in the social message, that this is now a normal legal product, may matter as much as the shops themselves.
Practical advice
If you are a parent or carer and this worries you, a few things are worth knowing. Signs that cannabis use has become a problem are not really about how often someone smokes. They are about what it is costing them. Look for school or college work slipping, giving up sport or hobbies that used to matter, money going missing, sleeping through the day, pulling away from friends and family, and becoming irritable or low when they cannot use. Start with a conversation, not an accusation. Young people who feel judged usually stop talking. Asking what they get out of it, and listening to the answer, tends to work better than a lecture. Your GP is a good first port of call and the conversation is confidential. In England, young people can also be referred to local drug and alcohol services, and many areas have services designed specifically for under-18s. The NHS website lists local services, and the charity FRANK offers free, confidential advice. If your child is already using cannabis alongside prescribed medicines, do tell a pharmacist or doctor. Cannabis can interact with some medicines, including some used for mental health, and the safest thing is for the professional looking after them to know the full picture. Finally, if you are worried about your own use rather than a child's, the same services are open to adults, and you do not need to be at crisis point to ask.
What to know
A large study of about 637,000 young people in Northern California found that diagnoses of cannabis use disorder in 12 to 17 year olds began rising after cannabis was legalised for adult use in 2016, reversing a fall in the years before. The study cannot prove the law change caused the rise. It is observational, and other things were changing at the same time, including the strength of cannabis products. The researchers' main message is about how we measure harm: surveys of how many teenagers use cannabis can miss a rise in the number who are actually being harmed by it. None of this changes UK law. But the signs of problem use are worth knowing, and help is available and confidential. Sources: Addiction (Wiley), 'Adolescent Cannabis Use Disorder Following Recreational Cannabis Legalization in California, USA: A Population-Based Interrupted Time Series Study', Padon et al., 15 September 2026, https://doi.org/10.1111/add.70597. News-Medical, 'Adolescent cannabis use disorder diagnoses rise after legalization', 15 September 2026, https://www.news-medical.net/news/20260915/Adolescent-cannabis-use-disorder-diagnoses-rise-after-legalization.aspx. Medical Xpress, 'Teen cannabis surveys may be missing rise in diagnosed disorder, study of 637,000 adolescents finds', 15 September 2026, https://medicalxpress.com/news/2026-09-teen-cannabis-surveys-disorder-adolescents.html. Kaiser Permanente Division of Research, 'Teen cannabis surveys may miss rise in use disorder', https://divisionofresearch.kaiserpermanente.org/teen-cannabis-use-disorder/. This article is for general information and does not replace advice from a doctor, pharmacist or other qualified healthcare professional. If you are worried about your own or someone else's drug use, please speak to your GP or a local drug and alcohol service.
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