The Negative Effects of Cannabis Use

Here on The Green Fund, we like to promote the positive effects of cannabis and the related health benefits. But not all of cannabis' effects are beneficial. Read this article to learn about some of the problems associated with cannabis use.

We note that the subject contained in this article represents illegal activity in certain jurisdictions. Whilst we do not condone any acts which are contrary to any such laws, we understand that readers in those jurisdictions which have decriminalised cannabis may find this article of interest.

The chances are that if you're reading this, you already know about the various health benefits that cannabis can provide. Possessing analgesic, anti-inflammatory and mood-lifting effects, cannabis has proven effective in managing numerous conditions, such as chronic pain, epilepsy, insomnia and depression, amongst others.

For all the pro-cannabis media that exists, you may be excused for thinking that it's some kind of wonder drug with minimal downside. But this may not necessarily be the case, as cannabis consumption is not without consequence.

With the landscape constantly changing in regards to cannabis legalisation, whether for recreational or medical purposes, more and more people are using cannabis products to treat ailments or for leisure. As such, it is important for us to understand ALL the effects of cannabis and not just the positive ones. After all, an informed consumer is better equipped to make sensible decisions about a product.

There is typically a lot of noise that surrounds the anecdotal evidence for the negative effects of cannabis. Even with research, some studies will report detrimental effects, while other similar studies will not. With this being the case, this article will focus on the areas where the evidence is the strongest.

Short-term Effects

If you have consumed cannabis before, then you are probably familiar with most of its acute effects. When a person consumes cannabis, any THC present in the plant is absorbed into the bloodstream via the lungs (in the case of inhalation) or the gut (in the case of ingestion). Alongside other cannabinoids, THC makes its way to the brain and other organs where it exerts a wide range of effects.

THC is psychoactive, so it can have a big impact on cognition and neural functioning. It alters the way we process information, which can impair our judgment. By changing the way information is processed in a part of the brain called the hippocampus, THC can also impair our ability to form new memories.

In high doses, THC can over-stimulate certain neural pathways, which can lead to states of paranoia and anxiety. This is why cannabis high in THC content is not recommended for inexperienced cannabis consumers.

Motor coordination and balance can also be affected, with cannabis slowing down most reflexes. When combined with impaired judgment, these effects can prove dangerous as they negatively affect our ability to complete tasks that require a high degree of motor functioning, such as driving a car or operating heavy machinery.

Alongside these effects, two of the most commonly reported side-effects of cannabis use are dry, red eyes and a sensation known as 'cottonmouth'. The red-eyes are caused by the blood vessels in your eyes dilating, while cottonmouth is caused by a reduction in saliva production. While they can be unpleasant, neither of these side-effects are particularly detrimental or harmful. 

Long-term Effects

While some of the short-term effects can be troublesome, most research is focused on the long-term effects of chronic or heavy cannabis use. This is largely because these effects are poorly understood and they are associated with a larger number of negative health consequences.


One of the first things people usually want to know about using cannabis is how addictive it is. Although many think of cannabis as non-addictive, there is evidence to suggest that long-term cannabis use can result in addiction. 

According to the DSM-IV (the Diagnostic and Statistical Manual of Mental Disorders, 4th Edition), 9% of the individuals who experiment with cannabis become addicted. This number increases dramatically if you observe only the individuals who began using in their teenage years or those who use cannabis daily.

The most recent version of the manual, the DSM-V, details a newly classified condition called Cannabis Use Disorder (CUD). This condition is characterised by a range of symptoms and behaviours, such as heavy or reckless cannabis use, neglecting responsibilities due to cannabis use, cravings, tolerance and withdrawal symptoms.

Irritability, sleeping difficulties, dysphoria and anxiety are just some of the troublesome symptoms one can experience with cannabis withdrawal. Any attempts at cessation can be difficult and may increase the likelihood of relapse.

Adolescents are the most at-risk group for the potential development of cannabis addiction. Research suggests that cannabis use during your adolescent years can influence multiple addictive behaviours in adulthood. 

Cannabis use during adolescence has been linked with a reduction of activity in dopamine neurons in the reward regions of the brain. These findings could explain the increased susceptibility to addiction and abuse later in life and are consistent in several studies

While these findings may support the idea of cannabis as a 'Gateway Drug', other drugs such as alcohol and nicotine can be classified in this way, as they exert similar actions. Some more likely explanations as to why cannabis may be a gateway drug are an individual's personality traits (e.g. addictive behaviours) and the increased exposure to other narcotics due to cannabis' legal status.

The Brain

There is great interest in the long-term effects of cannabis use on the brain, due to the plant's psychoactive effects. Research has shown that prolonged or heavy cannabis use, especially during adolescence, can cause changes in certain structures of the brain. Individuals who commence use during their adolescence are more at risk than those who start in their adult years, as the brain is still developing up until the age of 21.

Cannabis decreases neural connectivity in the precuneus and the fimbria, which greatly impairs their functions.  The precuneus is responsible for alertness and awareness, while the fimbria is a part of the hippocampus involved in learning processes and memory. Impaired function in these areas has a large impact on habits and routines and some have even pondered whether it contributes to a decrease in IQ.

In regards to mental health conditions, cannabis has often been reported to exacerbate the symptoms of conditions such as depression and anxiety. However, in a lot of cases, the causality of these symptoms is not established. There is even evidence (both anecdotal and clinical) to suggest that using cannabis actually improves these symptoms in some individuals. Cannabis has also been known to exacerbate the symptoms of schizophrenia sufferers and, in rare cases, has been linked with the onset of psychosis.

The link between cannabis use and the onset of mental illness often has many confounding variables, which makes causality hard to establish. In these cases, it is important to review all the factors that could lead a person to cannabis use and mental illness before attempting to establish a correlation.

The Lungs & Airways

Similar to tobacco smoke, cannabis smoke contains toxic chemicals that can irritate bronchial passages and the lungs. As such, smoking cannabis can result in a range of respiratory problems, such as airway inflammation, increased airway resistance, coughing and wheezing. It also increases the risk of developing a respiratory condition such as chronic bronchitis or COPD, while aggravating existing respiratory illnesses, such as asthma or cystic fibrosis.

Despite these effects on the lungs, the relationship between long-term cannabis use and the development of lung cancer is unclear. According to the National Institute of Drug Abuse (NIDA), there is no conclusive evidence that smoking cannabis can cause lung cancer and further research is needed.

Other Risks

As we mentioned earlier, cannabis use has a number of short-term effects that can impair driving but long-term use may also have an impact. Alongside alcohol, cannabis is the drug that is most frequently associated with driving accidents and fatalities. The overall risk of experiencing an accident doubles when a person commences driving soon after they've consumed cannabis. 

The risks of commencing cannabis use during adolescence have also been discussed but there are a few more reasons why this could be detrimental. Early cannabis use can affect cognitive functioning which can hinder academic performance and increase the risk of dropping out. Heavy use during adulthood is associated with lower income, unemployment, criminal behaviour and life dissatisfaction however, there are many factors that could influence these outcomes.

Long-term or heavy use of THC-containing cannabis may also lead to a condition called cannabinoid hyperemesis syndrome (CHS). This syndrome is characterised by persistent nausea and vomiting, stomach pain and dehydration. Depending on the severity, this can even result in kidney failure. Not all heavy cannabis consumers will develop this condition and the only known cure is the cessation of cannabis use.

Although there are numerous health benefits which cannabis is accredited with, further research is needed to fully understand the severity and impact of its side-effects. The future of cannabis as a medicine is dependent on how we are able to maximise these benefits while mitigating the associated risks.

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Josh Griffin
Josh Griffin

Josh is a Perth-based writer with a background in psychology and pharmacology. Through his studies he has gained an interest in abnormal psychology, mental health and psychopharmacology and has reported on these topics. Currently, his main focus is on cannabinoids and their medical potential.

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