The often contradictory effects of cannabis have proven to be a barrier that prevented it from gaining the mainstream medical approval that it has today. Nowhere is this more evident than in its effects on mental health.

Cannabis’s effect on mental health is incredibly convoluted, long, and expansive, and has been the topic of much research and discussion. This means that all that could be said about the topic couldn’t fit into this short overview.

According to archaeological finds, humans have been consuming and perhaps smoking cannabis for at least 5,000 years, possibly longer. Cannabis cultivation for other purposes may have an even longer history. Its short-term psychoactive properties are well-known and generally regarded as pleasant. In the vast majority of usage cases, the most obvious effects will only last for a few hours, and possibly a day. However, the different compounds of cannabis can stay in a system for weeks.

For normal recreational users who may consume some cannabis every few weeks or months, it’s generally agreed that there are no long-term mental health effects. However, for regular users, such as heavy recreational users and those who use medical cannabis for specific symptoms, there are a number of risks involved.


Cannabis has long been recommended as an effective treatment for anxiety symptoms. However, in a very small number of users, it has been reported to trigger the same. A meta-study from 2014 linked anxiety to cannabis use, concluding that there was a “small” risk. However, many of the studies analyzed also fail to conclude that cannabis caused anxiety in subjects, leading to the possibility that those with anxiety are merely more likely to use it.


So-called “psychotic illnesses” have been linked to cannabis. Links between schizophrenia, schizoaffective and bipolar disorder and the extremely heavy use of cannabis have been found by different studies. However, the risk even to heavy users is relatively low.

In the case of schizophrenia, cannabis use can double one’s risk. However, in context, the risk is quite small, with it being 7 in 1,000 for non-users and 14 in 1,000 cannabis users. ( Like with studies on anxiety in depression, there is still also the possibility that the pre-existing condition increases cannabis use, rather than cannabis use creating the condition.

However, there is also evidence that shows smoking cannabis after one has recovered or adjusted from a psychotic illness can cause a relapse in many cases. This means close monitoring is important when consuming cannabis if one has a history of these episodes.

Effects on developing brains

Our brains do not fully develop until we are in our early adulthood, which means the brain may not be fully formed until one is 18 or possibly much later in the mid-to-late 20s. There is strong evidence to suggest that heavy cannabis before the brain has matured can greatly increase the risk of stunted brain development and psychosis in later years. Depression and anxiety are also far more likely in those who were heavy cannabis users in their teens.

Today’s more potent strains are also causing concern. Modern cannabis is much more likely to have permanent effects on developing brains compared to the much milder cannabis available before the 1980s, given that modern cannabis is cultivated specifically to have large doses of THC.

As with alcohol, it’s important to avoid or moderate the use of cannabis, especially before one’s 20s, perhaps even later. However, the relative risk remains quite small, even compared to alcohol and controlled use is definitely key to avoiding any of the serious long-term risks.

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