Around 500,000 Americans have died from opioid overdose in the past 20 years, according to the Centers for Disease Control and Prevention (CDC). An important feature of opioid dependence is the development of tolerance, which is different from addiction but closely related. Here’s what you need to know about how opioids work, what tolerance is, and how tolerance is related to opioid addiction.
What Are Opioids?
Opioids are drugs either derived from the opium poppy plant or created synthetically in a laboratory. They mimic your body’s own endogenous opioids, which are naturally-occurring chemicals that regulate pain and give a sense of well-being by aiding in the release of endorphins and dopamine.
While opioids are used medically to regulate pain, they are also frequently abused because they create a sense of euphoria, according to the Merck Manual. The terms “opioid crisis” or “opioid epidemic” are used to describe the startling rise of opioid addiction and overdose in the United States during recent decades.
What is Opioid Tolerance?
When someone has developed a tolerance, increasing amounts of a drug are needed to produce previously-felt effects. In repeated opioid use, tolerance is common. “When an opioid is used on a regular basis, the brain adjusts so the effects of the opioid become less over time—this is tolerance,” Tucker Woods, DO, an Addiction Medicine Specialist and Chief Medical Officer at Restorative Management Corp, tells WebMD Connect to Care.
Tolerance is created through changes in the brain’s opioid and dopamine receptors. When the brain is continually flooded with opioids and dopamine, the body responds by decreasing the amount of opioid and dopamine receptors on cells’ surfaces. The result is that higher levels of the drug are needed to get the same effect, according to the National Institute on Drug Abuse (NIDA).
NIDA also says that tolerance is the cause of many opioid overdoses. A person with opioid tolerance will become accustomed to high doses. If they stop using the drug, tolerance reverses and the brain’s receptor levels return to normal. When the same person then takes the same amount of drug they used to take, they can easily overdose.
What’s the Difference Between Opioid Addiction and Tolerance?
Tolerance is not addiction, but it is related. The two are linked because people with tolerance can feel driven to take more of the drug to get the same euphoric effect, according to Mayo Clinic.
Another difference is that tolerance describes a biological phenomenon, whereasthe term "addiction" describes a complex disease in which tolerance is a common feature. Sometimes called “substance use disorder”, addiction is considered both a brain disorder and a form of mental illness, according to NIDA.
“Unlike tolerance, addiction is a disease of the brain whereby someone cannot stop using a substance (opioids in this example) despite harmful consequences,” says Woods.
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