Drug users are notorious for making poor decisions, but a new study suggests that there may be a scientific explanation for their bad judgement. Apparently, regular cocaine and methamphetamine use causes changes in the brain that may hamper a user’s ability to discern right from wrong.

For the study, which is now published in the online journal Psychopharmacology, researchers from the University of New Mexico and the nonprofit Mind Research Network investigated how the neural networks and brain functions of chronic cocaine and methamphetamine users in United States jails affected their ability to evaluate moral situations or scenarios. To do this, the team analyzed the life history of substance abuse of 131 cocaine and meth users, and 80 non-users incarcerated in New Mexico and Wisconsin prisons. The inmate volunteers also had their brains scanned while they were asked to make moral decisions.

Results revealed that, when compared to the non drug users, inmates who had regularly used stimulants had abnormal neural activity in the frontal lobes and limbic regions of their brains during the moral processing. These areas of the brain are used for evaluating emotions. In addition, the lifetime drug users also had less activity in the amygdala, an area of the brain associated with regulating and understanding emotions. Lastly, the drug-using inmates also exhibited less activity in their anterior cingulate cortex, yet another area of the brain heavily associated with the regulation of emotions. All together, these results suggest that years of drug abuse have damaged the areas of users’ brains related to decision making, and in turn hindered their ability to accurately tell right from wrong.

In this situation, it’s hard to know for certain which came first: the drug abuse or the poor decision skills. The team acknowledged that it could be that people who are prone to regular stimulant use may have already struggled with moral processing even before they began to use drugs. However, the brain scans still show the severe effect that long-term drug use can have on the brain.

It’s not just the brain that takes a toll from regular long-term drug abuse. Meth and cocaine are stimulant drugs, meaning they increase alertness, attention, and energy. A lifetime of stimulant abuse has serious effects on the human body that reach far beyond compromising the user's moral compass. For example, meth is known to accelerate the cellular aging process, which can cause stark physical changes in a methamphetamine addict’s physical appearance. In addition, long-term meth abuse can cause weight loss, severe tooth decay and tooth loss, and skin sores. Meth abuse can significantly increase one’s chance of suffering from a stroke and developing Parkinson’s disease.

The effects of cocaine abuse aren’t much more pleasant. According to the National Institutes of Health , long-term cocaine use can lead to increased irritability, restlessness, panic attacks, paranoia, and in some cases full-blown psychosis, a serious mental disorder characterized by thinking and emotions that indicate a person has lost contact with reality.

Source: Fede SJ, Harenski CL, Schaich Borg J, et al. Abnormal Fronto-Limbic Engagement in Incarcerated Stimulant Users During Moral Processing. Psychopharmacology. 2016.