to the nervous system and internal organs. Moreover, illicit drug uses are commonly linked with aplastic anaemia, bone marrow repression and variety of systemic disorders [3]. Hence, early diagnosis of these disorders and identification of the complications they result in is important. Haematology is the branch of medicine that deals with blood, blood-forming organs and blood disorders. This article summarizes current information on the haematological changes that are observed in different substance use disorders and the utility of these changes in clinical assessment. Overview of Substances Alcohol Alcohol is the most commonly used worldwide. Alcohol consumption is one of the leading causes of death. Chronic alcohol intake can interfere with various physiological, biochemical and metabolic processes of the blood cells and affect multiple organ systems. Alcohol use, especially in heavy drinkers, can cause different metabolic derangements. Haematological adverse effects of acute and chronic alcohol use result from both direct and indirect effects [4,5]. The direct consequences include toxicity to the blood-forming organs (viz. bone marrow); the blood cell precursors; and the mature Red Blood Cells (RBC's), White Blood Cells (WBC's), and platelets, resulting in fewer than-normal or non-functional mature blood cells. Alcohol's indirect effects include metabolic or physiological alterations resulting in liver disease and nutritional deficiencies such as folate deficiencies that