What Is Opioid Tolerance?
Opioids are a class of medication used in the treatment of acute and chronic pain associated with injuries, surgeries, or long-term medical conditions such as cancer or palliative care. Opioid tolerance, most simply put, occurs when a patient who has taken opioids for a prolonged period begins to require ever-increasing amounts of the drug to achieve pain relief.
But why, exactly, does tolerance occur?
There are two types of tolerance. Innate tolerance is a genetic predisposition to drug insensitivity, and this type of tolerance is usually identified early on when the medication the patient receives offers little or no pain relief.
Acquired tolerance is the consequence of repeated drug exposure, and is usually the cause of opioid tolerance. It can occur via a few different mechanisms.
Pharmacodynamic tolerance occurs when the drug has been in the body for so long that it becomes accustomed to it, and therefore, less sensitive to its presence in the body. This typically happens after prolonged use of the drug.
In some cases this decreased response may be acute, occurring when the brain and central nervous system enact processes to immediately lessen the effects of a substance. This means that in many cases the effects of the drug will be mitigated by the reduction of receptors in the brains of each specific substance.
Pharmacokinetic tolerance occurs when smaller amounts of the drug are reaching the target sites. This can be caused by an increase in the introduction of enzymes required to break down the drug.
The last class of acquired tolerance involves learning. Behavioral tolerance occurs when a person learns to function despite repeated exposure to the drug. For example, chronic alcohol users may not present with motor impacted from being intoxicated, because they have, over time, learned adaptive motor function in response to the impairment.
Conditioned tolerance is another subclass of learning, in which cues are linked to drug use. For example, drug-using behavior (such as using a needle) becomes a signal that predicts drug exposure to the body. In other words, eventually, the very act of preparing to take a drug triggers an anticipatory response.
~ G. Nathalee Serrels, M.A., Psychology