Tolerance occurs when the initial dose of a substance loses its effectiveness over time and a person needs more drugs to produce the desired effect.
There is evidence that structures in the brain change and are damaged by abuse that makes addiction possible. Neurochemicals in the body adjust to substance abuse beyond just tolerance. With tolerance, it takes more and more to get that high, when a person is addicted, their bodies crave and need those substances to function. Even though a person may not “feel” drunk, alcohol will still damage the brain as more is consumed. While drug abuse is a bad habit, addiction is defined more as a compulsive disorder, where a person’s risk versus reward perception is almost completely turned off.
Often we hear people with addictions talk about getting high just to feel normal. A person who needs more and more drugs or is tolerating their use better is experiencing a symptom of addiction or substance use disorder.
Tolerance and withdrawal are very important symptoms for which to be on the lookout because they are two pieces that demonstrate that drug abuse is crossing the line and turning into an addiction. And, the psychological symptoms that accompany withdrawal, such as depression and anxiety, may be mistaken as simply part of withdrawal instead of an underlying mood disorder that requires independent treatment in its own right.
Cross-tolerance is also dangerous because addiction to one substance means the person is more likely to struggle with addiction to other drugs. By building a tolerance to one drug, one may develop a tolerance to a drug with a similar chemical layout even if they have never ingested the drug.
So drugs and alcohol are working alright. Just not the way you might want them to.