Introduction

Depression is one of the most common mental health disorders, affecting an estimated 5% of the adult population worldwide. Depression and substance use disorders are highly comorbid, meaning that they often occur within one individual, and consequently, having one puts you at a higher risk of the other.¹ This correlation can arise for many different reasons, from self-medication to escape from difficult life experiences. Cannabis has been shown to alleviate some symptoms of depression, but has also been coined a “gateway drug” to more harsh illicit substances. Therefore, an issue arises for clinicians choosing to prescribe medical cannabis for the treatment of depression, as chronic use of cannabis has been linked to the development of other substance use disorders.² 

Understanding depression and its link to substance use disorders

A bit about depression 

Depression can affect anyone at any age. It is characterised by feelings of sadness for weeks or months, along with other symptoms, such as:

• Feelings of unhappiness or hopelessness
• Losing interest in things that were once pleasurable or important
• Fatigue and sleeping problems
• Lack of sex drive
• Changes to appetite
• Aches and pains
• Suicidal thoughts
• Symptoms of anxiety

Because depression is a highly complex disorder of the brain, its exact causes are not well understood. However, certain factors make an individual more susceptible to it. These include:³

• History of family conflict
• Bullying
• Stressful or traumatic life events
• Coming from a low-income household
• Sex
• Genetics

Some of these factors can also increase an individual's risk of developing a substance use disorder.

Depression and substance use disorders

Depression and substance use disorder are at a high risk of both being present in one individual; they are highly comorbid. Some researchers speculate that individuals with depression may seek out substances as a way to self-medicate. This comes from changes in the processing of rewards in the brain, which underlie both disorders.⁴ This means that in the brains of those suffering from depression and substance use, perceived rewards are handled differently. Hence, changes to the brain that occur with depression make it easier to get a substance use disorder and vice versa.

Other researchers believe that these two arise from specific risk factors. Things like genetics, trauma, and stress can cause changes to the brain that make it more likely to experience depression, substance use disorders, and many other mental health disorders.⁵

Medical cannabis use in depression

Cannabis has been shown to improve symptoms of depression in some individuals. For instance, it significantly improves scores on the  PHQ-9 questionnaire. This questionnaire asks about the frequency of experiencing depression symptoms, spanning from feelings and emotions to appetite and how these symptoms affect everyday tasks.⁶

With this being said, there is not enough evidence to prove the effectiveness of medical cannabis in the treatment of depression symptoms across populations, and cannabis is not to be considered a cure for depression or other psychiatric disorders.⁷ Regardless, medical cannabis continues to be prescribed to those with depression in countries all around the world.^{6, 8}

Substance use disorders and medical cannabis

Is cannabis a “gateway drug” or a harm reduction tool?

Cannabis has been associated with increasing the risk of developing substance use disorders of other substances.² It has long been regarded as a “gateway drug”; a drug which, despite being relatively safe itself, can lead someone to more addictive and more dangerous drugs. 

On the other hand, cannabis has been previously used to substitute for more dangerous and addictive drugs. An example of this is using medical cannabis as a substitute for opioids in those with opioid use disorder.⁹ However, it is important to note that scientific studies of cannabis as a medical treatment for anything are still limited.¹⁰

Risk of substance use disorder in individuals with depression 

As those with depression already have an increased risk of developing a substance use disorder, it may not seem wise to prescribe them with depression medical cannabis for their symptoms. Cannabis may have the potential to exacerbate substance abuse tendencies in some individuals who are most at risk genetically and environmentally.

There is also the added layer of developing cannabis use disorder from medical cannabis. As depression increases the risk of substance use disorders, it also increases the risk of developing cannabis use disorder.¹¹

Managing risks and optimising outcomes

Depression, medical cannabis, and substance use disorders are all closely connected. The key to safe and effective treatment is close monitoring by a medical professional. It is important to make sure that those who are particularly vulnerable to substance use disorders are looked after when being prescribed medical cannabis for depression.

Below are some things to consider if you are taking medical cannabis for depression and want to minimise the risk of developing substance use disorders:

• Prolonged and frequent exposure to cannabis leads to tolerance. This increases the risk of dependency on cannabis, hence can increase the risk of developing other substance use disorders in the future² 
• Cannabis products can vary in their chemical compositions. THC is the main addictive chemical in cannabis. Higher concentrations of THC in the product can result in a greater risk of cannabis dependence or the development of substance use disorders¹²
• The way by which you administer medical cannabis (smoking, ingesting oils, vaporising, etc) can influence your use habits and make it more or less likely to develop a substance use disorder¹³
• Therapies which are known to be effective in the treatment of depression (for example talking therapies) should be used to address its underlying causes to prevent increasing the risk of substance use disorders

If you are someone with depression and use medical cannabis, it is crucial to have regular monitoring of your medications. It may also help to build a support circle of family and friends or support groups to have trusted people in your life who can observe any changes or signs of substance use disorders.

Summary

Depression increases the risk of developing substance use disorders. This is because of shared causative factors like stress, trauma, or genetics, but also can occur due to specific changes which occur in both disorders.

Medical cannabis has been used to treat symptoms of depression in some countries and was shown to be effective for some individuals in promoting better sleep, improving mood, and increasing general well-being. However, cannabis has also been regarded as a “gateway drug”, with many studies showing that frequent cannabis use can lead the user to have an increased risk of developing other substance use disorders.

To minimise the risk of developing substance use disorders in those with depression using medical cannabis to manage symptoms, it is recommended that any cannabis prescription is closely monitored in the most vulnerable individuals.

Additional ways of minimising the risk of developing a substance use disorder as a patient include: avoiding high THC-containing medical cannabis products and undergoing other treatments for depression (such as talking therapies) to address the underlying cause of depression and minimise time on medical cannabis.