BPD, like other mental illnesses, can cause abnormalities in brain chemistry. For example, positive chemicals such as serotonin or endorphins are required to feel happy, calm, content, etc. A client with BPD may produce far less of the required chemicals needed to maintain level thinking.
This effect often leads to drug and alcohol abuse to feel happy, confident, and a bevy of other emotional states a BPD-affected client may not usually feel. Drug and alcohol use combined with mental illnesses is viewed as a form of self-medication.
In addition, self-isolation and negative thoughts can make daily tasks unbearable; alcohol and drugs may be used as a form of escape.