Tolerance and Dependency: Causes and Symptoms

Tolerance and dependence are two concepts often discussed under the heading of addiction. There is, however, often confusion about what these words mean and how they are involved in drug abuse and addiction. Articles and discussions of addiction often use “dependent on” and “addicted to” interchangeably, which can be inaccurate. What tolerance is, how it happens, and its role in addiction can also sometimes be confusing.

Clarifying tolerance and dependence, and how they differ from and relate to addiction, can help people who are struggling with drug abuse and their loved ones understand the various physical and mental elements of addiction, how it develops, and in some ways, how it is treated.

What is the Meaning of Drug Tolerance?

Tolerance is defined by the National Institute on Drug Abuse as a condition where the body needs an increased amount of the drug to experience the same effect that was experienced when the person first started taking the drug. In other words, when tolerance has developed, it makes the person feel like the drug isn’t working as well as it used to. In many cases, this makes the person want to take more of the drug in order to have the same effect as before.

How Does Drug Tolerance Develop?

Merck Manuals explain that tolerance develops when a person has been using a drug regularly for a long time. When a substance changes the way neural pathways work, the brain’s first reaction is sometimes to counter that substance’s behavior. So, as an example, if a person is taking opiates that act to increase dopamine levels in the brain, the brain will, over time, decrease dopamine production in response. The result is that the individual feels less of the drug’s effect at that dose.

When tolerance occurs, the individual may act on the desire to use more of the drug. Whether the drug is illicit or medically prescribed, this is misuse of the drug that constitutes abuse. The risk of increasing the dose is that the person will also become tolerant to the higher dosage. In turn, this may cause the person to increase the amount used again. This can create a spiral of increasing dosage and developing tolerance that leads to dangerously high use of the drug, resulting in a heightened risk of overdose. Sometimes, it can also cause or contribute to dependence on the drug.

What is Dependence?

To be dependent on a drug is to be unable to function normally without it. A person who has asthma, for example, may be dependent on an asthma medication to be able to continue to breathe normally. Similarly, a person with chronic pain may have a dependency on a painkiller to be able to perform daily tasks without excessive pain. This type of drug dependence is typically in response to an illness or injury that has already occurred, in which case the drug is prescribed to return the person to a healthy level of functioning.

In contrast, dependency on a drug may sometimes develop after having used or abused it for weeks, months, or years. In the painkiller example above, dependence may develop after the person has used a medication for an acute injury or illness, causing the person to feel the need to continue using the drug to avoid pain or other bad feelings even though the condition that originally required the drug has resolved. In other cases, as described by the National Institute on Drug Abuse, recreational drugs can have a similar effect, leading to dependence after long-term use. The person may use the drug to experience feelings of pleasure; however, after long-term use, the person may no longer be able to feel pleasure without taking the drug.

This type of dependence manifests through the person experiencing withdrawal symptoms if the drug is stopped – symptoms that make it difficult or impossible for the person to stop using the drug without drastic effect on the individual’s ability to function.

How Does Dependence Occur?

A study from Neuropsychopharmacology describes the process of moving from initial drug use to drug dependence. Dependence can develop in a similar way to tolerance and can sometimes be a result of tolerance. The individual whose brain is producing less dopamine because of long-term drug use will find that stopping use of the drug causes unpleasant feelings, such as

  • Depression or anxiety
  • Fatigue and insomnia
  • Nausea
  • Muscle discomfort

When dependence has developed, the person must continue taking the drug to avoid these discomforts.

Similar effects can occur with other brain pathways. A person using a stimulant may become dependent on the drug to have energy or be productive while a person using a depressant may be dependent on it to avoid anxiety or agitation. In some cases, such as with alcohol or benzodiazepines, this dependence can become serious enough to risk the person’s life if the drug is stopped too abruptly, due to severe physical responses to loss of the substance.

What is the Difference Between Drug Abuse and Dependence?

The National Institute on Drug Abuse provides a simple clarification on the differences between tolerance, dependence, and addiction. The key concept to take away is that neither tolerance nor dependence alone constitutes addiction, which is a diagnosable mental health disorder. However, tolerance and dependence affect one another, and, when combined, can contribute to the development of addiction. Ultimately, the difference is that addiction is a mental health disease or condition while tolerance and dependence are physical responses to drug abuse.

At its most simple level, addiction means a person has become unable to stop using a drug even when faced with negative consequences, such as mental or physical health challenges, relationship issues, or other problems caused by the drug use. Instead, the person feels compelled to continue using the substance and cannot control or stop use.

Tolerance and dependency can both develop without leading to addiction. Similarly, addiction can develop without the person experiencing tolerance or dependency. However, when tolerance, dependence, and addiction all manifest together, it can be extremely difficult to stop drug use without intervention.


How Does Drug Dependence Affect Addiction Treatment?

Dependence or tolerance can require special care for people who are in treatment for drug abuse or addiction. For example, if a person is dependent on the drug, the withdrawal process is medically supervised, involving tapering down from the drug so the body has time to adjust to losing the substance by increasing its own production of neurochemicals again. This makes it easier for the person to stop using the drug without severe withdrawal symptoms that can derail recovery or cause harm to the individual.

If a person has become tolerant of high doses of the drug, this can pose a risk if the person relapses to drug use. As explained in a study from the British Medical Journal, many overdoses occur when a person has been in recovery long enough to lose tolerance and then relapses, taking the high dose that was used before treatment. Because of this, people who have developed a high tolerance may require extra post-treatment support to stay motivated to avoid relapse.

How is Drug Dependence Treated?

Tolerance, dependence, and addiction are all treated in professional, reputable drug treatment programs. The therapies that are offered through research-based rehab programs take into account how these conditions affect the person’s response to treatment and make sure the individual has the greatest possible chance of recovering from substance abuse.

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