There is a profound co-occurrence of addiction and mental illness. Both addiction and mental illness have life-altering effects on those affected. Excessive use of medications undoubtedly harms one's body. Addiction and side effects of drugs (such as hallucinations and paranoia) take a toll on an individual's psychological health, while social conditions often worsen as the need for drug use outweighs the need to maintain relationships. Similarly, individuals suffering from a mental health crisis not only experience psychological symptoms, but many find that their physical health is compromised (e.g. loss of appetite, sleep disturbance), along with their standing in society due to anxiety or stigma associated with the disease. This student next asked the question: Does mental illness lead to addictive behaviors, or does extensive use of addictive drugs lead to mental health problems? This essay will first define dependency, then explore both sides of the issue, and finally suggest how to address these issues in Ireland. The addictive substances will be alcohol and cannabis, while the main psychological pathologies will be depression, schizophrenia and bipolarism. To complement this student's major, this essay will focus on adolescents. Addiction is a substantial problem worldwide; Compton et al. (2007) found that 7-10% of people experience illicit drug abuse or dependence in their lifetime. Addiction was defined by the American Psychological Association (2014) as a condition in which the body requires a drug to avoid both physical and psychological withdrawal symptoms. Westreich (2009) argues that this is not simply pathological health promotion in Ireland; particularly because studies have shown that Irish teenagers experience more mental disorders than their peers in other countries. This essay explored whether mental illness was a trigger for taking addictive substances or whether excessive use of such substances led to mental health problems. A detailed argument was presented focusing on cannabis and alcohol addiction. It is concluded that there is a link between addiction and mental illness, furthermore both can be considered a risk factor for the other. The focus throughout the essay was on adolescence: the teenage years were found to be the most vulnerable for both mental illness and substance abuse. This student believes resources need to be invested in promoting positive mental health among Irish adolescents to reduce the risk of future addictive behavior in the next generation.