Addiction is a brain disease commonly associated with mental illness, defined as ‘a state in which an organism engages in compulsive behaviour, even when faced with negative consequences’. Drug addictions are extremely difficult to quit, and always have a risk of relapsing whilst becoming sober, due to the brain having to be returned to its original state.
The use of drugs commonly begins due to being in a social environment surrounded by individuals who already use drugs, leading to an internal/external pressure to begin to use them too. Another reason may be that individuals are chasing the feeling of euphoria to distract themselves from their current mental state and reality, and both situations can quickly lead to a downward spiral if not controlled.
According to Skinner’s theory, the actions of an individual are mainly decided by reinforcement they previously receive; reinforcing stimuli. After an action is carried out, they receive a response (positive or negative) and, depending on this response; the individual is either more or less likely to want to repeat this action again. Drug addicts are an example of this theory as they are constantly chasing the positive reinforcements experienced after taking a drug, leading to a long-lasting repetition of this action. These positive reinforcements are a result of increased dopamine production.
Essentially, drugs interact with your neurons, altering the chemical activity of the brain.
Different regions of the brain correspond to different functions, connected by neuronal pathways which transport electrical impulses between the regions. As shown in the diagram, the frontal cortex is responsible for pleasure/euphoria and the positive sensations experienced when receiving a reward; this is the region which is activated when a drug is taken.
Dopamine is a neurotransmitter formed from the amino acid tyrosine. Once the neurotransmitter-containing vesicles are released they migrate across the synapse before binding to dopamine receptors on the opposing neuron. Carrier proteins then act as pumps to remove dopamine molecules from the receptor surface, allowing the level of dopamine to be controlled. This is crucial as abnormalities can lead to disorders and diseases such as depression or Parkinson’s disease. 
Long term drug use leads to the reward system of the brain becoming over stimulated leading to heightened danger-related emotions, causing anxiousness and worry when not taking drugs. This is a vicious cycle as the brain will feed on the positive reinforcement (dopamine) but continuously increase its threshold level for pleasure (due to decreased sensitivity), causing an increase in the frequency of drug use but effectively causing the threshold to increase each time. This makes addiction recovery difficult as the brain must be reprogrammed to function at a lower dopamine level. 
Different drugs affect the brain in different ways. Marijuana is a drug known for causing relaxed and light-headed sensations, which occurs by its interaction with the endocannabinoid system, specifically the cannabinoid receptors. The cannabinoid receptors are responsible for short-term memory, coordination, learning, and problem-solving. THC (a cannabinoid present in cannabis) mimics the structure of anandamide (a natural cannabinoid), meaning it is also able to bind to the cannabinoid receptors. 
The THC cannabinoid is metabolised before binding to the cannabinoid receptors, particularly in areas where they exist at a high density (hippocampus, cerebellum and basal ganglia). The hippocampus is the region of the brain responsible for long term memory meaning the binding of THC to the receptors interferes with memory storage leading to both short and long term memory loss. This interference can also be experienced via the cerebellum (lack of coordination) and basal ganglia (temporary impaired muscle movement). 
There are also drugs such as amphetamine and cocaine which prevent the carrier proteins from pumping excess molecules away from receptors, leading to a surge of neurotransmitters being released and allowing a more intense high to be experienced. This surge of neurotransmitters leads to cocaine being a common drug in people with mental illnesses.
Unfortunately, mental health disorders and addiction are often co-occurring disorders, and it can sometimes be difficult to tell which occurs first – it is reported that around 50 per cent of substance abusers also have a mental illness. 
Those who originally only suffer from mental illnesses may refer to drugs to experience increased moods and decrease existential, negative thoughts. They constantly chase this sensation which leads to constant and increasing drug use, and eventually addiction; i.e. drugs are used as a form of self-medication. Contrarily, those with drug addictions may decrease the sensitivity of their receptors to dopamine, which can lead to mental illness.
It is important that integrated treatment is used to treat both conditions simultaneously, as both have a high risk of relapsing and can cause a knock-on effect on the other condition.
It is also crucial that anyone who suffers from addiction receives emotional support such as group therapy to discuss any emotions which led to them using drugs in the first place. Drug addiction recovery is difficult but definitely not impossible, and it requires equal levels of effort from both the patient and doctor. The patient must be comfortable enough with their sober state of mind to prevent them from turning to drugs again, and this will take some time.
We must abolish the stigma around addictions as they are harmful stereotypes which can lead to undeserved embarrassment in substance abusers. This may lead to them not reaching out for professional help which decreases their quality of life inevitably. There are many text lines such as shout which are open to substance abusers for advice and support and can be a starting point for their healing journey.
- ‘Drugs, Brains, and Behavior: The Science of Addiction’, National Institute On Drug Abuse, July 2020
- Dr Vincent Berger , ‘Famous Psychologists: B.F. Skinner’, Psychologist Anytime Anywhere, 2005
- ‘Teaching Addiction Science’, National Institute On Drug Abuse
- Phil Newton, ‘What Is Dopamine?’, Psychology Today, Apr. 26, 2009
- Kevin Bonsor & Nicholas Gerbis, ‘How Marijuana Works’, howstuffworks
- ‘The Connection Between Substance Use Disorders and Mental Illness’, National Institute On Drug Abuse, April 2020