Substances of abuse
Most of today’s street drugs started off as naturally occurring substances that our ancestors found by accident. Refining of those substances created the first illicit drugs. All addictive drugs strongly stimulate dopamine(Drug information on dopamine) neurotransmission,11 through which they stimulate the reward circuit as well as learning, memory, and behavioral circuits.12 People vulnerable to addiction will experience these drugs as highly pleasurable.13 This perception can lead to a spiral of poor choices and negative consequences.
Increased vulnerability to addiction
When adolescents with a vulnerability to addiction experiment with drugs or alcohol(Drug information on alcohol), they may stumble on a substance so rewarding that it effectively reverses their dysphoria. Depending on the degree of genetic, experiential, and physiological vulnerability, this experience can drive recurrent use, misuse, abuse, or in some cases, full-blown addiction.14
The more powerfully and rapidly a drug stimulates dopamine neurotransmission, the more reward/euphoria it creates and by extension, the more addictive it is. When smoked, crystal methamphetamine can multiply dopamine neurotransmission 150 times. When snorted, cocaine can multiply transmission 35 times. Even eating chocolate can double the rate.15
Comorbid addiction disorder and ADHD is frequent. The high heritability of ADHD and addiction seen in twin and adoption studies suggests that there are common genes between the two. Other studies suggest that there may be numerous genetic insults that increase vulnerability to ADHD and addiction. No single gene has been shown to make more than a small contribution.16
A number of studies have found that the risk of ADHD is higher in persons who both carry genetic variants and have been exposed to an environmental risk factor (eg, prenatal maternal smoking, alcohol consumption, psychosocial adversity).17-19
Functional MRI studies show reduced frontostriatal circuit activity in both ADHD and substance use disorder,20 specifically in the dorsal ACC—a part of the brain that monitors behavior, suppresses emotional feelings (eg, cravings), and chooses among competing priorities. This reduction in activity worsens with chronic drug use.21
Current data suggest that these genetic, environmental, and drug-induced insults combine to create an area of impaired dopamine neurotransmission. This combination is associated with diminished perception of reward, worsening cognition, and impaired behavioral inhibition. This makes life seem boring, worsens the ability to recognize future negative consequences, and diminishes the ability to say “no” to an enticing distraction. This explains how a severely addicted person’s ability to choose among competing priorities degenerates to the point where he chooses to use drugs rather than pay the rent or buy food.