Substance abuse is a health problem with serious psychological and psychiatric dimensions and multiple social and economic consequences. Cancer is a disease that threatens not only life and physical integrity but mental health as well. Oncology patients suffer from mental disorders in high rates, especially from depression and anxiety. The role of substance abuse in the pathogenesis of cancer is studied systematically, since there are research data supporting the mutagenic effects of certain substances. It has been supported that a possible dysregulation of the immune system is linked to the oncogenic processes induced by substances of abuse. Specifically, opioids are the first addictive substances that have been identified as oncogenic factors. However, conflicting results have been offered by experimental animal studies, which showed that opioids, such as morphine, depending on the dosage administered, may not only enhance the process of tumor growth, but also inhibit it. Additionally, research data indicate that the use of cannabis may be associated with cancer, either as an independent factor or in relation to other mutagenics, although it is not yet clear to which extent these effects may be connected to the disease, especially once the consumption of tobacco and alcohol by these patients are taken into account. However, it has been argued that certain cannabinoids may have biological -anticancer- activities which could be used therapeutically without being accompanied by the corresponding 9-tetrahydrocannabinol psychoactive effects. It is well known that alcohol is a risk factor for developing head and neck cancer, and epidemiological studies indicate that the higher the consumption of alcohol, the more mortality due to cancer increases. In addition, it is suggested that there is no safety level for alcohol consumption regarding the risk of developing cancer; that is even a minimum daily consumption is associated with the occurrence of certain types of cancer. Specific components have been identified in tobacco, which are considered to be carcinogenic and responsible for tumor development in various sites. Moreover, complicated psychiatric problems arise due to substance abuse in cancer patients, either in the context of pain treatment, or under pre-existing dependence. The rational use of opioid analgesics, when it is medically required as suggested by the health professionals specialized in the treatment of acute pain in cancer patients, could be a therapeutic option. Substance abuse reduces treatment compliance, worsens cancer prognosis and seems to be a negative factor for the quality of life of these patients. Current literature highlights the importance of appropriate psychiatric interventions to address substance abuse in cancer patients.