As of 2021, more than 36 million people worldwide were estimated to be experiencing substance abuse disorder. This condition spans a spectrum of patterned drug use that causes issues in a person’s life. At the more severe end of the spectrum is substance addiction. To understand why some people are more susceptible to addiction and why it can be so difficult to treat, let’s take a look at how addictive drugs affect the body.
When someone repeatedly uses an addictive substance, their brain may adjust to account for its regular presence. This is called tolerance; it diminishes the drug’s effect and means more is required to produce the same experience. Alcohol, for instance, increases the transmission of chemical messengers like endorphin and GABA, which promote sensations of pleasure and calmness. When someone uses alcohol frequently, their brain will adapt to its presence. So when they don’t use it, their brain receives fewer signals from those pleasure- and calm-inducing neurotransmitters, which impacts their energy and mood. In addition to the unique effects each addictive substance has, all of them alter the release of dopamine in a brain region called the nucleus accumbens. This area is part of the brain’s reward pathway, which is sensitive to experiences that give us pleasure and drives us to repeatedly seek them out. The reward pathway is essential to our well-being— but addictive substances also exploit it.
When someone is repeatedly using a substance and their body has adapted to its presence, they may develop dependence where the drug is necessary for them to function comfortably. Meanwhile, repeated use can decrease the influence of the brain’s cortex, which is responsible for driving deliberate decision-making and limiting impulsive behaviors. And it can increase the influence of the brain’s subcortex, which is crucial in habit-learning and impulsivity. Together, these changes can make someone feel a lack of control over how they’re using a substance. This can mean suffering in other facets of their life and taking risks to continue using it. If the substance leaves their system, their body's equilibrium is disrupted, so they may experience cravings that motivate thoughts and behaviors of seeking and using the drug. And if they continue without the substance, they may experience withdrawal. Opioids, for example, relieve pain and induce sedation. Withdrawal from them causes heightened pain, anxiety, and insomnia.
The faster a drug reaches the brain and stimulates the reward pathway, the more addictive it is. For example, heroin and morphine have similar effects, but heroin’s chemical structure enhances its ability to penetrate the blood-brain barrier and bind to specific receptors. This makes it faster acting and more addictive than morphine. Meanwhile, compared to nicotine patches and gum, regular and electronic cigarettes are more addictive because smoking and vaping deliver nicotine to the brain fastest.
Depending on their chemical structure and mode of delivery, drugs vary in how addictive they are— but people also vary in how susceptible they are to substance use disorder. Scientists think this is due to a mix of life experiences and genetically inherited traits, though neither predetermines addiction. Trauma and mental health conditions appear to make people more susceptible. And, overall, using addictive substances before 18 is considered a strong risk factor. This is in part because the reward pathways of younger brains are especially sensitive.
Many of the genetic factors at play remain unknown, but some genes do seem to track with certain substance use disorders. For example, specific genes increase the risk for nicotine addiction by making certain receptors more sensitive to nicotine and withdrawal from it more difficult. At the same time, some genes actually make people more resistant to substance use disorders, such as genes that slow the breakdown of alcohol, leading to unpleasant side effects that make people less interested in drinking. In fact, one drug that treats alcohol use disorder operates with this very mechanism.
Detoxing from a substance the body has become dependent on can be extremely difficult. And in some cases, abruptly quitting can also be physically risky, so it isn’t always advised. Beyond detoxing, quitting is often a long-term process. It’s aided by treatment plans that prioritize addressing underlying conditions; developing new associations with experiences previously linked with drug use; and creating safe, supportive environments. Ultimately, both addiction and recovery are the results of a brain with an incredible ability to adapt from experience.
