Tolerance and Dependence

Tolerance and dependence are two important physiological consequences of drug and alcohol use.  Although they often go hand in hand with addiction, it is helpful to distinguish the three terms.

Tolerance results when the body adapts to a substance; what was initially an effective dose needs to be increased in order to produce the desired result.  This is why, for example, habitual coffee drinkers can drink 4-5 cups a day without getting the jitters that a non-coffee drinker would experience.  In the case of most drugs of abuse, which target the brain’s reward system, tolerance develops rapidly as the brain decreases both its production of dopamine (the neurotransmitter associated with pleasure) and the amount of available dopamine receptors.  Because of these physiological changes in the brain, drug users must take higher and higher doses to achieve that original feeling of euphoria, and they can become desensitized to other natural pleasures.  People who have built up a high tolerance for drugs or alcohol can consume more of a substance without appearing to display its effects—a heavy drinker may consume large amounts of alcohol without appearing to be drunk, for example—but the substance is still wreaking havoc on their bodies.  One dangerous side effect of tolerance occurs when someone in recovery relapses and consumes drugs or alcohol in the amount that they had previously been accustomed to.  Their newly “clean” body may not be as tolerant as it once was, and people can accidentally overdose this way.

Like tolerance, physical dependence can occur with the repeated use of any substance.  When a person becomes physically dependent on a substance, suddenly stopping that substance will cause symptoms of withdrawal because the brain has chemically adapted to it.  Withdrawal symptoms vary predictably according to the drug involved.  Coffee drinkers, for example, frequently experience headaches when they miss their morning coffee.  Nicotine withdrawal symptoms include shakiness, sleep problems, irritability, and increased appetite; and alcohol withdrawal symptoms include shakiness, sweating, nausea and anxiety.  When someone who routinely takes prescription painkillers abruptly stops taking them, he or she may undergo restlessness, diarrhea, vomiting, pain, and other withdrawal symptoms.  In this case, if the person is under the care of a physician, the solution is to taper off slowly so that the brain’s chemistry can adjust.  Sometimes a doctor will even prescribe another drug to help to counteract the effects of severe withdrawal. 

Although tolerance and dependence are frequent signs of addiction, a person may experience physical dependence and tolerance to a certain substance and not be addicted to it.  Addiction is distinguished by loss of behavioral control, perseverance in a certain behavior despite its negative consequences, and psychological preoccupation or obsession.  This may be confusing because the term dependence is sometimes used interchangeably with addiction.  For this reason, it can be useful to distinguish between physical and psychological dependence.  For example, people who take potentially addictive prescription medicines and strictly follow their doctors’ instructions will most likely develop a certain degree of tolerance and physical dependence, but they will not generally become addicted unless they also possess other risk factors for addiction.