Pleasure and reward systems are two of those.
“If I were to wipe that dopamine pathway out of your brain, you'd never feel good, life would be rotten and you'd probably commit suicide,” Hanson said.
Repeated use of the same drug can lead to a tolerance in some individuals, causing them to gradually increase their dose to achieve the desired effect.
And once the mind is hooked, the body soon follows in the form of withdrawal symptoms, which commence soon after an addict's last high begins to wear off.
Withdrawal from opiates, for example, can include “restlessness, muscle and bone pain, insomnia, diarrhea, vomiting, cold flashes with goose bumps and involuntary leg movements,” according to the National Institute on Drug Abuse.
Because of the mental and physical characteristics of addiction, many scientists and other health authorities say it's a chronic, complex disease.
According to the institute's website, the disease typically begins to develop during adolescence.
An institute report states that during these key developmental years, a young person's brain is still in the final stages of growth.
“The prefrontal cortex — located just behind the forehead — governs judgment and decision-making functions and is the last part of the brain to develop,” the report states.
“This may help explain why teens are prone to risk-taking, are particularly vulnerable to drug abuse, and why exposure to drugs at this critical time may affect propensity for future addiction.”
Hanson said scientists are studying the relationship between genes and addiction because there are roughly 100 genes associated with addiction.
“This means that the cause can vary from person to person,” he said. “If we know what the cause is, we can be more selective with our treatments and likely more effective.”
Another reason scientists and researchers are studying the link between genes and addiction, Hanson says, is to allow medical professionals to be more proactive when dealing with drug and alcohol addiction.
“It is likely that this information would make our prevention strategies more targeted and more likely to succeed,” he said.
Scientists also are studying addictive behavior in mice to help identify which genes are involved in the process of becoming addicted.
Mice and humans are very similar, genetically, and mice exhibit varying levels of addiction in lab tests.