Our powerful brains give us the intellectual capacity to quickly figure out that something induces pleasure, quickly decide we want more of it, and quickly work out how to get it. Luckily, we also have higher-brain regions in place to mitigate or overrule such base impulses as, “Thing make me feel nice, must get more thing.” These impulse-control centers aren’t perfectly understood but are most probably located in the prefrontal cortex, along with other complex cognitive functions.19 Regardless, impulse control allows us to curb our excesses and recognize that descending into pure hedonism is not a good idea overall.
Another factor here is the plasticity and adaptability of the brain. A drug causes excess activity of a certain receptor? The brain responds by suppressing the activity of the cells those receptors activate, or shutting down the receptors, or doubling the number of receptors required to trigger a response, or any method that means “normal” levels of activity are resumed. These processes are automatic; they don’t differentiate between drug and neurotransmitter.
Think of it like a city hosting a major concert. Everything in the city is set up to maintain normal activity. Suddenly, thousands of excitable people arrive, and activity quickly becomes chaotic. In response, officials increase police and security presence, close roads, buses become more frequent, bars open earlier and close later, and so on. The excitable concert-goers are the drug, the brain is the city; too much activity and the defences kick in. This is “tolerance,” where the brain adapts to the drug so it no longer has the same potent effect.
The problem is, increased activity (in the reward pathway) is the whole point of a drug, and if the brain adapts to prevent this, there’s only one solution: more drug. An increased dose is needed to provide the same sensation? Then that’s what you use. Then the brain adapts to that, so you need a bigger dose. Then the brain adapts to that, and on it goes. Soon, your brain and body are so tolerant of a drug that you’re taking doses that would legitimately kill someone who had never tried it before, but all it does is provide the same buzz that got you hooked in the first place.
This is one reason why quitting a drug, “going cold turkey,” is so challenging. If you’re a long-term drug user, it’s not a simple matter of willpower and discipline; your body and brain are now so used to the drug they’ve physically altered to accommodate it. Sudden removal of the drug therefore has serious consequences. Heroin and other opiates provide a good example of this.
Opiates are powerful analgesics that suppress normal levels of pain by stimulating the brain’s endorphin (natural painkilling, pleasure-inducing neurotransmitters) and pain-management systems, providing an intense euphoria. Unfortunately, pain exists for a reason (to let us know about harm or damage), so the brain responds by increasing the potency of our pain-detection system, to cut through the blissful cloud of opiate-induced pleasure. So users take more opiates to shut it down again, and the brain strengthens it further, and so on.
Then the drug is taken away. The user no longer has something that made them incredibly calm and relaxed. What they do have is a super-enhanced pain detection system! Their pain-system activity is strong enough to cut through an opiate high, which for a normal brain would be agonizing, as it is for a drug user going through withdrawal. Other systems affected by the drug are similarly altered. This is why cold turkey is so hard, and legitimately dangerous.
It would be bad enough if it was just these physiological changes that drugs cause. Alas, changes in the brain also alter behavior. You’d think the many unpleasant consequences and demands of drug use should logically be sufficient to stop people using them. However, “logic” is one of the first casualties of drug use. Parts of the brain may work to build tolerance and maintain normal functioning, but it’s so diverse that other brain areas are simultaneously working to ensure we keep taking the drug. For example, it can cause the opposite of tolerance; drug users become sensitized to the effects of a drug by suppression of the adaptation systems,20 so it becomes more potent, compelling the individual to seek it out even more. This is one factor that leads to addiction.?
There’s more. Communication between the reward pathway and the amygdala serves to provide a strong emotional response to anything drug related, aka “drug cues.”22 Your specific pipe, syringe, lighter, the smell of the substance, all these become emotionally charged and stimulating in their own right. This means drug users can experience the effects of a drug, directly from the things associated with it.
Heroin addicts provide another grim example of this. One treatment for heroin addiction is methadone, another opiate that provides similar (though reduced) effects, theoretically enabling users to give up gradually without going cold turkey. Methadone is supplied in a form than can only be swallowed (it looks like worryingly green cough syrup), whereas heroin is usually injected. But so strong a connection does the brain make between injection and the effects of heroin, that the act of injecting causes a high. Addicts have been known to pretend to swallow methadone, then spit it into a syringe and inject it.23 This is an incredibly dangerous act (if only for hygiene reasons) but the warping of the brain by drugs means the method of delivery is almost as important as the drug itself.
Constant stimulation of the reward pathway by drugs also alters our ability to think and behave rationally. The interface between the reward pathway and the frontal cortex, where the important conscious decisions are made, is modified, so that drug-acquiring behaviors are prioritized above normally more important things (such as holding down a job, obeying the law, showering). By contrast, negative consequences of drugs (being arrested, getting a nasty illness from needle sharing, alienating friends and family) are actually suppressed in terms of how much they bother or worry us. Hence an addict will shrug nonchalantly at losing all their worldly possessions but will repeatedly risk their own skin to obtain another hit.
Perhaps most disconcerting is the fact that excessive drug use suppresses activity of the prefrontal cortex and impulse-control areas. The parts of the brain that say, “Don’t do that,” “That’s not clever,” “You’ll regret this,” and so on—their influence is diminished. Free will may be one of the most profound achievements of the human brain, but if it gets in the way of a buzz then it’s got to go.24