New 20-year study: long-term use of cannabis raises risk of mental health problems

A new study on marijuana was reported in the UK Daily Mail.


A definitive 20-year study into the effects of long-term cannabis use has demolished the argument that the drug is safe.

Cannabis is highly addictive, causes mental health problems and opens the door to hard drugs, the study found.

The paper by Professor Wayne Hall, a drugs advisor to the World Health Organisation, builds a compelling case against those who deny the devastation cannabis wreaks on the brain. 

Professor Hall found:

  • One in six teenagers who regularly smoke the drug become dependent on it
  • Cannabis doubles the risk of developing psychotic disorders, including schizophrenia
  • Cannabis users do worse at school. Heavy use in adolescence appears to impair intellectual development
  • One in ten adults who regularly smoke the drug become dependent on it and those who use it are more likely to go on to use harder drugs
  • Driving after smoking cannabis doubles the risk of a car crash, a risk which increases substantially if the driver has also had a drink
  • Smoking it while pregnant reduces the baby’s birth weight

Last night Professor Hall, a professor of addiction policy at King’s College London, dismissed the views of those who say that cannabis is harmless.

‘If cannabis is not addictive then neither is heroin or alcohol,’ he said.

‘It is often harder to get people who are dependent on cannabis through withdrawal than for heroin – we just don’t know how to do it.’

Those who try to stop taking cannabis often suffer anxiety, insomnia, appetite disturbance and depression, he found. Even after treatment, less than half can stay off the drug for six months.

I’m seeing a lot of “Christian libertarians” being supportive of initiatives to legalize drugs, but there is a reason why drugs are illegal. They hurt people. They hurt families. Why would we want to celebrate and normalize something that is known to cause harm?

Please see below for more recent studies on the effects of marijuana.

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4 thoughts on “New 20-year study: long-term use of cannabis raises risk of mental health problems”

  1. “Why would we want to celebrate and normalize something that is known to cause harm?”

    That’s a straw man. Not everyone who wants to see the size and scope of government severely decreased is a member of NORML or named Snoop Dog.

    There is no doubt in my mind that all of the drugs we have classified as schedule A drugs are harmful, particularly if abused. This is also no doubt in my mind that alcohol or many things that we eat have little or no redeeming quality, particularly for the Body of Christ. Should we ban all of them? How does that bring people to Christ? How does that open the door for the Holy Spirit?

    I would rather recognize that we live in a fallen world that cannot be reformed but can be saved through Christ, than to legitimize the use of physical violence against people who think there is no harm in smoking a joint or shooting some heroin or drinking a bottle of jack or gambling their every penny away or shacking up or cheat on their spouse. I will call them out individually; I will pray that the Holy Spirit convicts them and they repent; I will shout it from the mountain tops that they are harming themselves; but I will not send armed men to coerce them when a loving rebuke is what they need. Now, if they do bring harm to others, either through maliciousness (burglary to support their habit) or negligence (leaving a child unattended because they are blitzed) then I will gladly use the state to protect the innocent from further harm if we can.


  2. BTW, let’s talk about this study. It is simple correlation. I think we can intuit some causation such as smoking cannabis probably does make you a worse student since it is psychotropic. On the other hand, the group of cannabis users are self selective and a simple correlation does not prove that cannabis causes mental illness. It could be that people who are prone to mental illness are more likely to start using cannabis and other drugs, perhaps in an attempt to alleviate some of their symptoms. The only way to prove this would be to have a randomized study where use is not self selective but I believe that would be evil.


  3. Not to be a punk, but if someone didn’t already know that long term use of a mind-altering drug was bad for your health, it’s not really a matter of ignorance (in the lack-of-information sense), it’s a problem of hubris. People who deny that drugs are bad for them don’t want to recognize a) that societal limitations on their behavior are rational and completely proper, and b) that even if they can deny or disobey the societal limitations, that there are natural limits on their behavior. These people are so heavily invested in their own egos, they are offended at the mere suggestion that someone or something is going to impose a consequence on their behavior.

    As for drug wars, or use of force, there is nothing inherently wrong with a society criminalizing drugs, but there is a danger in a police power gone too far. I’d be happy simply applying a drug test for state-benefits. You are ineligible for state benefits if you have certain substances in your blood. Do whatever foolish thing you like, just don’t make the rest of us pay for your foolishness.


  4. I do not use pot nor any illegal drugs, but I think it is time to call a halt to the disastrous “drug wars” that have ravaged our country’s “soul” for many decades. Especially where marijuana is concerned. If people want to self-destruct, ain’t nothing you can do about it. Meanwhile the drug wars have squandered hundreds of billions of dollars of national wealth, corrupted and militarized law enforcement, made a mockery of the Constitution and destroyed tens of thousands of lives and livelihoods. Far more grief and destruction than the illegal drugs themselves have caused. At this point there are only two groups that benefit from pot remaining illegal: the “drug lords” whose livelihoods would be destroyed by legal marijuana. The others are law enforcement – especially the DEA – who would see their power and budgets shrink.


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