Substance use disorders involve a pattern of using drugs more extensively than intended and continuing use despite harmful consequences. This includes legal substances like alcohol and nicotine, as well as illegal drugs. These disorders often involve both physical and psychological dependence, reflecting compulsive use of substances that significantly alter thoughts, feelings, and behaviors, contributing to a major public health issue.

Understanding the concepts of physical dependence, psychological dependence, and tolerance is crucial in the context of substance use disorders. Physical dependence refers to a state where the body has adapted to the presence of a drug. Over time, this adaptation becomes the norm, requiring continued use of the drug to function properly and avoid withdrawal symptoms. Withdrawal symptoms, often the opposite of the drug's effects, can include pain, cramps, and severe agitation, depending on the substance.

Psychological dependence involves a perceived need for a drug driven by emotional or psychological desires. This form of dependence, while lacking the stark physical symptoms of physical dependence, can be equally compelling and difficult to overcome. It often involves cravings and a strong desire to use a drug to achieve a sense of well-being or escape from stress.

Tolerance, closely linked to physical dependence, occurs when the initial dose of a drug loses its effectiveness over time, compelling the user to consume higher doses to achieve the same effect. This escalation can lead to overdose, as the body's ability to cope with the drug's toxic effects is overwhelmed.

Drugs impact the brain's reward system, primarily through the neurotransmitter dopamine, which enhances feelings of pleasure and well-being. The brain's adaptation to these increased levels of dopamine during drug use leads to changes in brain structure and function, particularly in areas involved in reward, stress, and self-control. This neurological framework is fundamental in developing and maintaining substance use disorders.

The distinction between these types of dependencies and their interplay with the brain's reward systems helps explain why overcoming addiction is not merely about willpower but involves comprehensive behavioral and physiological understanding and interventions.