There has been much research and interest over the years into the effects that cannabis has on our mental health and cognitive abilities.
A great deal of attention has focused on whether cannabis use leads to psychotic symptoms and schizophrenia, but it has also been claimed that cannabis use can lead to depression and other mood disorders.
People diagnosed with a mood disorder such as depression and bipolar disorder experience symptoms such as a low mood, feelings of sadness, displaying apathy toward most activities and tasks, agitation, fatigue, low self-esteem, and an inability to concentrate on anything for a sustained period.
Other symptoms related to depression include weight loss, insomnia, suicidal ideation, and loss of energy.
However, a persistent low mood tends to be the strongest indicator of depression and each person may exhibit different combinations of other symptoms.
Research seems to suggest that people diagnosed with mood disorders are particularly vulnerable to dying prematurely and committing suicide. (1,6)
Bipolar disorder is characterised by the tendency for people to alternate between a depressed state and feelings of mania reflected in an improved mood, and an increased positive attitude where they can appear to be engaging with day-to-day life with sustained enthusiasm. (6)
After alcohol, cannabis is the most consumed drug in the UK and is known to be widely used by adolescents as they seek to experiment with drugs to experience the pleasurable effects associated with psychoactive drugs. (1a)
Cannabis, in one form or another, has been taken by people for centuries, originally people’s motivation to consume cannabis was purely medicinal but as time progressed and people started to become aware of its properties more people began to take the drug recreationally.
Cannabis is not viewed as harmful as other drugs such as opioids or benzodiazepines although it must be stressed that it still is a psychoactive drug that can alter our brain chemistry. (2)
Cannabis can be found in extracts of the Cannabis Sativa plant, which was originally cultivated in Eastern Asia, but which can now be found further afield. Marijuana is the name given to the dried flowers and leaves of the plant. (4,5)
The leaves from the Cannabis Sativa plant are generally smoked although they can be mixed in with food and eaten.
Cannabis contains around 500 different chemicals and not all of them have been tested individually to assess the effect that they have on the human brain.
Over 50 of these are chemically similar to THC but no firm evidence is available to establish whether any of these have a causal effect on the mental health of users. (8,9)
In the UK there are primarily three varieties of cannabis, these being cannabis resin, marijuana, and sinsemilla (which includes skunk).
The main psychoactive constituent in cannabis is THC which is responsible for generating the positive effects that people experience when taking the drug. (8)
Anyone taking cannabis will experience its effects almost instantly and its effects can last for up to 4 hours depending on the type and amount a person has taken.
Despite being less harmful than other drugs cannabis is capable of causing physical and psychological dependence in users.
Cannabis produces feelings of pleasure and relaxation, and regular users may find that they will soon develop a tolerance to the drug, meaning that they will need to take more of the drug to experience the same effect that they had when initially taking the drug.
Cannabis users can find themselves tied into a very strong psychological dependence as the pleasure and relaxation they feel when taking the drug is a strong motivating factor to continue taking the drug. (5)
Consumers of cannabis are likely to also experience a difference in the way they perceive time, and their visual and auditory sensory experiences may be altered.
It has also been established that cannabis users can experience feelings of depersonalisation represented by their sense of detachment from themselves and their surrounding environment.
Cannabis has been popular among artists and musicians due to the notion that it has been reported to boost creativity. (4,8)
These changes can lead to significant long-term problems as regular consumption of cannabis for a lengthy period can have a detrimental effect on human cognition and mental health.
During the 1990s scientists identified the ECS in the human brain which appears to be responsible for governing internal brain processes relating to mood, anxiety, and stress.
The endocannabinoid transmitters found in the brain are similar to those chemicals present in the Cannabis Sativa plant. (9)
This means there are receptors in the brain that chemicals in cannabis likeTHC can attach themselves to and start to take effect by altering our brain chemistry (this is how psychoactive drugs work).
Everyone has an ECS in their brain regardless of whether they take cannabis or not. (9)
This does not mean that everyone will experience depressive symptoms though as there are many individual biological and psychological differences between people that determine who will experience depressive symptoms and who will not. (9)
Many research studies have found that cannabis use can lead to poorer mental health in many individuals, but some studies have found that cannabis use does not negatively impact mental health at all so there is no strong research consensus to draw on. (2,9)
Not all research studies use the same methodology and there are differences in the participants that are used which have not been accounted for in all of the studies.
Other factors need to be considered when investigating the association between cannabis use and depression/poor mental health, as just because there is an association between cannabis use and depression it does not mean that cannabis use directly causes depression. (2,9)
Medical studies have shown that THC and the element CBD in cannabis can generate anti-depressant-like effects in some users who have depressive symptoms but only at certain dose levels. This suggests that there is a complex relationship between cannabis and depression. (9)
The Royal College of Psychiatrists states that cannabis use does contribute to mental health problems in individuals that were previously well and exacerbate the symptoms of those already diagnosed with a mental health condition.
The younger you are when you take cannabis the greater the risk to your mental health.
Many people take cannabis to alleviate their anxiety and low mood, but research indicates that this will only improve their condition for a short time and does not tend to help the condition in the long term. (7)
If people continue to use cannabis to self-medicate and do not seek medical help for their depression and/or anxiety symptoms, then their situation may get worse in the long term compared to if they did not take cannabis at all.
It has been proposed that regular, heavy cannabis use can cause depression in two ways, firstly becoming intoxicated on cannabis can reduce a person’s ability to psychologically adjust to the ever-changing demands of life.
Cannabis does negatively affect many of our cognitive abilities such as paying attention, maintaining concentration, planning, problem-solving, and memory.
Anyone who has a reduced capacity in aspects of their psychological functioning will find it hard to meet the challenges of studying, finding and holding down a job, forming relationships, and managing their finances.
Not being able to achieve the things they want to in life can begin to make someone feel depressed. (2,9)
Secondly, it has also been stated that high amounts of THC can affect the levels of serotonin in a person’s brain and lead to depressive symptoms.
However, it is worth adding that there are many forms of cannabis, and each form will contain different levels of THC. For example, anyone consuming skunk will be taking in higher levels of THC than someone smoking cannabis resin. (2)
So research indicates that it is cannabis doses containing a higher percentage of THC that are more likely to cause depressive symptoms.
THC has been identified as the key psychoactive ingredient in cannabis that plays a key role in the sought-after euphoric and relaxing sensations (which leads to psychological dependence) people receive when taking the drug. (2)
However, research has indicated that the THC component in cannabiscan exacerbate the negative symptoms of those who have already been diagnosed with depression.
It would appear that THC does behave like a typical psychoactive drug as it leads to both physical and psychological dependence.
Many research studies have sought to investigate whether cannabis use can be a catalyst for suicidal intentions, there have been mixed findings in these studies, so it is difficult to say with any certainty that cannabis use does make people who take the drug more vulnerable to developing suicidal intentions.
Due to the simultaneous increase in cannabis use and suicide figures in young males, there has been a tendency over the years for social commentators to assume that it is cannabis use that is responsible for the increase in suicide numbers in young males. (2)
However, after careful analysis, researchers have found that this is not necessarily the case as these young males can share other factors apart from substance misuse which can cast doubt on whether cannabis does make users more prone to suicide.
Research conducted over the past 10 years has indicated that brain functions and behaviour are more vulnerable to the effects of THC during teenage years than at other stages in the lifespan.
This would explain why the link between cannabis use and poor mental health/depression appears to be stronger when a person starts consuming cannabis during their adolescent years.
This could be down to the fact that the human brain is going through a critical stage of development during the adolescent phase and consuming psychoactive drugs during this time can significantly impair brain development.
Ironically this is the time when adolescents seek to begin experimenting with drugs as they approach early adulthood. (2)
It has been noted in certain research studies that cannabis use leads to a decrease in people’s motivation, indicated by a display of apathy towards most areas of life.
This can be exhibited by lacking enthusiasm, having poor concentration, a lack of determination and persistence, and no ability to initiate action to get tasks completed.
This behaviour has been recognised as a symptom of depression although people do not necessarily need to exhibit this symptom to be diagnosed with depression.
This is because there is a multitude of symptoms that are associated with depression, only a few of which need to be present for a diagnosis to be established. (9)
Scientific investigation has found that many cannabis products available on the international market are far more potent compared with 20 years ago, with THC levels in certain cannabis forms making up 25% of the drug.
As THC is the main psychoactive ingredient in cannabis it is almost certain to have more of an impact on the brain chemistry of a greater number of cannabis users than it previously did, which will see a rise in the number of associated mental health problems. (10)
It is important to establish that just because there is a link between cannabis use and depression it does not mean that smoking cannabis causes someone to become depressed, it just means that they are associated in some way.
Association and cause and effect are not the same thing. (2,3)
Three explanations have been offered to account for the apparent link between cannabis use and depression.
The psychoactive compound THC has a detrimental effect on our brain chemistry leading to a low mood. This has been proposed as one explanation although the research findings are not always clear for this to be stated with any certainty.
The psychoactive compound THC has a detrimental effect on our brain chemistry leading to a low mood. This has been proposed as one explanation although the research findings are not always clear for this to be stated with certainty.
This approach proposes that there is no causal link between cannabis use and depression but that those who consume cannabis and those who are prone to depression have certain things in common.
These factors may be biological (genetics), personality, or environmental as individuals who use substances and have poor mental health share certain risk factors (e.g. social deprivation).
There is a strong relationship between cannabis use and depression and scientific studies have failed to produce any consistent findings.
This is down to the fact many factors need to be accounted for when examining the relationship between cannabis use and depression and not all research studies have used the same methodology or investigated the same form of cannabis when conducting their research.
(1) Black, D., Grant, J. (2013) DSM5 Guidebook: The Essential Companion to Diagnostic and Statistical Manual of Mental Disorders, 5th Edition. APP. London.
(1a) Hughes, L. (2010) Cannabis Use and Psychosis. In Phillips, P., McKeown, O. Sandford, T. Dual Diagnosis: Practice in Context. Wiley-Blackwell. Chichester.
(2) Degenhardt, L et al (2012) The association between cannabis use and depression: A review of the evidence in Castle, D., Murray, R.M., D’Souza, D. (ed) Marijuana and Madness. Cambridge University Press. Cambridge.
(3) Gross, R. (1996) The Science of mind and behaviour. Hodder and Stoughton. London
(4) Kahan, M. (2014) Physical Effects of Alcohol and Other Drugs. In Herie, M. & Skinner, W. (ed) Fundamentals of Addiction: A Practical Guide for Counsellors. CAMH. Canada.
(5) Moss, A, Dyer, K (2010) The Psychology of Addictive Behaviour. Palgrave McMillan. New York.
(6) Pratt, C., Gill, K., Barrett, N., Roberts, M. (2007) Psychiatric Rehabilitation (Second edition). Elsevier. London
(7) Royal College of Psychiatrists (2022) How does cannabis affect mental health. Available?@ Cannabis and mental health – for young people | Royal College of Psychiatrists (rcpsych.ac.UK)
(8) Solowij, N. (1998) Cannabis, and Cognitive Functioning. Cambridge University Press. Cambridge.
(9) Stoner, S. (2017) Effects of marijuana on mental health. Alcohol and drug abuse institute, University of Washington. Available @ 2017mjdepression.pdf (uw.edu)
(10) Slade et al (2012) Is Cannabis becoming more potent? in Castle, D., Murray, R.M., D’Souza, D. (ed) Marijuana and Madness. Cambridge University Press. Cambridge.