There’s the very simplistic answer that states the truism that when people are in extreme emotional, spiritual or physical pain (all of which can be true for people suffering from mental illnesses), anything that ‘numbs’ the pain they are in will be a welcome relief.
In his book ‘Emotional Intelligence’, author Daniel Goleman says the following:
‘those who stay with the habit, becomingly increasingly dependent on alcohol or drugs, are using these substances as a medication of sorts, a way to soothe feelings of anxiety, anger or depression…those most vulnerable to addiction seem to find in the drug or alcohol an instant way to soothe emotions that have distressed them for years.”
Goleman then goes on to explain links between the underlying mental issue or illness, and the drug of choice, as follows:
ANXIETY = alcohol. Anxiety is often linked to a lack of a neurotransmitter called GABA, which is experienced as high levels of physiological tension. Alcohol eases that feeling of tension.
BUT alcohol is terrible for depression, as it tends to worsen it after a short ‘lift’.
So DEPRESSION = cocaine, and other stimulants. (BTW, Prozac et al is in the same ‘drug’ family as cocaine, just not with the same obvious instant ‘high’).
To quote Emotional Intelligence again:
“One study found that more than half the patients being treated at a clinic for cocaine addiction would have been diagnosed with severe depression before they started their habit, and the deeper the preceding depression, the stronger the habit."
(You’ll notice that cocaine didn’t ‘cure’ the depression, it just masked the symptoms…)
CHRONIC ANGER = heroin and other opioids.
“In a study of 400 patients being treated for addiction to heroin and other opioids, the most striking emotional pattern was a lifelong difficulty handling anger and a quickness to rage. Some of the patients themselves said that with opiates they finally felt normal and relaxed.”
A more alternative health view
So far so good, but I just wanted to throw a couple more ideas into the ring:
Depression, anxiety and chronic anger are all symptoms of PTSD, where a person continues to have a physiological ‘stress’ reaction to a previous (or ongoing) experience of acute or chronic trauma.
This post talks more about PTSD:
what is post traumatic stress disorder (PTSD)?
But a permanently switched-on, physiological, ‘fight or flight’ stress response is underneath a lot of the issues causing mental illnesses as described above, and that are causing people experiencing the fall-out from these issues to turn to drugs.
Secondly, the following quotes come from this excellent research document on the healing effects of meditation and self-compassion:
Bridging Practice and Science
“…feelings of love, warmth and caring, and the bonds between individuals that these feelings reflect, result from the release of beta-endorphins in the brain. These molecules are members of the opioid family, which underlie and reinforce the rewarding effects of food and drugs. As such, it’s been suggested that beta-endorphins originally evolved to reinforce social bonds, crucial for survival. A biological mechanism that encouraged individuals to form groups would promote survival…If feelings of warmth and caring are indeed associated with beta-endorphin release, it seems reasonable to suggest feelings of compassion…are also related to the natural release of these opioids.”
“…opioids are the most powerful painkillers known. Thus, if opioids are released naturally when one is in a state of compassion, it’s quite possible that the person would be less afflicted by their own physical, emotional or even vicariously experienced pain…Compassionate behavior [could] lead to a kinder society through increased desire and motivation to help, [and] the individual being compassionate would [also] suffer less.”
This scientific evidence suggests the following link between mental illnesses and drug use:
People develop mental illnesses because they don’t experience enough compassion and caring. They then turn to drugs and food to try to ‘replicate’ the chemicals that would be released naturally (ie, opioids), if they had more compassion and caring in their lives.
There’s a whole lot more to say about this, but it’s a very interesting idea, and jives very nicely with the other scientific research out there exploring WHY people get depressed, angry, self-absorbed, anti-social and anxious in the first place.
You can read more of that scientific background yourself at the following post:
'What does the latest scientific studies say about what causes personality disorders?'
Recreational drugs also cause mental illness
I forgot to mention this on Quora, but 'recreational' drug use has also been strongly linked to developing mental illnesses.
The following quote comes from this post on Mad in America:
"The Cannabis and Psychosis Awareness Project, a four-year study from Canada that was released on Tuesday, finds that smoking marijuana - particularly heavy use in early adolescence - is associated with a 40% increased risk of psychosis."