Many of today's drugs, both legal and illegal, can be traced back thousands of years. Drugs such as beer and wine date back at least 8,000 years, and traces of Ephedra vulgaris, a plant associated with cannabis, have been found in archaeological sites in Asia (Hamilton, King & Ritter, 2004). According to Ryder, Walker, and Salmon (2006) drug use is a common and normal act in today's society and it would be deviant from the norm to find an individual who has not used drugs. There is no specific definition for the term drug, as this would only cause debate among people (Ryder et al., 2006). This academic paper will focus on psychoactive drugs, which act on the central nervous system to alter states of consciousness by modifying mood, perception, and behavior (Ryder et al., 2006; Hamilton et al., 2004; Burton, Westen, & Kowalski, 2009).A model is a practical way to explain why events occur the way they do, models can describe a health problem and explain why and how the problem occurs (Ryder et al., 2006) . Models can also predict what is likely to happen, but more importantly, they can be used to plan interventions to solve a health problem (Ryder et al., 2006). This essay will describe the public health model and discuss why drugs do not have a fixed, predictable effect. Numerous models have been developed to understand drug use and its effects; the public health model integrates the perspectives of other models and focuses on three main factors (Ryder et al., 2006; Hester & Miller, 2003; Clapp & Shillington, 2001). The first factor considered is the host, referred to as the individual drug user (Hester & Miller, 2003; Ryder et al., 2006). The second factor is the agent, … at the heart of the article … of drug use (Ryder et al., 2006). The public health model emphasizes the importance of environmental factors that can encourage or discourage drug use (Hester & Miller, 2003). In conclusion, drugs have been used for as long as historians can remember drugs that alter people's mood and personality. an individual are called psychoactive drugs. The public health model integrates different drug consumption models to focus on three important factors. The first is the agent factor which focuses on the pharmacological aspects of a drug. The second is individual factors, which target physiological domains; age, weight and sex. Including psychological domains, what are the beliefs and expectations? Finally, the environmental factors that encourage or discourage drug use. The three factors mentioned above all influence the unpredictable nature of drug use.