Decision-making can be negatively impacted by drugs and alcohol. One of the problems with addiction is that it builds over time. A person addicted to drugs or alcohol can have a steady employment or be a productive member of society, leading to them believing that he or she may not have a problem or that it's "not that bad." In reality, millions of Americans are dependent on alcohol or drugs to make it through the day. A "good" person can easily get addicted to drugs or alcohol over time by becoming dependent on them. Some common situations are through pain-killers after an accident or surgery; some may overuse the pain-killers, or do not need to take as many as are prescribed, and as a result, get addicted to using pain-killers to make it through the day.
One way drugs and alcohol impact decision-making is through denial. People with addictions believe they are drinking a normal amount or taking a normal amount of pills to get through the day and are in denial about their addiction level.
Studies, like "Emotion, Decision-Making and Substance Dependence: A Somatic-Marker Model of Addiction" by Verdejo-Garcia et. al., show that substance dependence creates an impairment in decision-making that is similar to brain lesions. In his research, Verdejo-Garcia noticed that those who are dependent on substances, like alcohol or drugs, will take an immediate reward despite severe negative future consequences; like eating an entire bag of Halloween candy knowing you will feel sick afterwards, or taking drugs or drinking to excess despite knowing the medical impact. His study shows that decision-making depends on brain functions that regulate homeostasis and emotions, and dependency on drugs or alcohol impair these functions and lead to poor decision-making. Therefore, addiction creates situations where the individual is not able to make good decisions concerning their addictions. The article is linked below for further exploration.
The part of the brain that is responsible for spur-of-the-moment-decision-making is the orbitofrontal cortex, the part that is impacted by substance abuse. Researcher Joshua Jones, who studies the impact of the orbitofrontal cortex on decision-making, found that
... drug addiction is marked by severe deficits in judgment and bad decision-making on the part of the addict. We believe that drugs, particularly cocaine, affect the orbitofrontal cortex. They coerce the system and hijack decision-making.
Drugs and alcohol, therefore, impact an individual's ability to make good decisions when it comes to immediate responses to the system. This, coupled with denial about addiction, as well as the ease and availability of drugs and alcohol, mean that more and more Americans are at risk of becoming addicted.