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Updated on July 24, 2025
If you or a loved one is diagnosed with a medical condition, it’s often helpful to understand what is happening in the body as a result of that medical condition.
Opioid use disorder (OUD) is a medical condition characterized by the chronic use of opioids, and the continued use of opioids despite negative consequences to physical health, mental health, and quality of life. People with OUD also experience tolerance and withdrawal, requiring greater amounts of opioids over time, and experiencing distressing symptoms if they stop taking opioids or reduce the amount they are taking.
OUD can accurately be described as a chronic brain disease. While the causes of OUD are complex—and OUD results from a combination of multiple factors—the condition is closely linked to dysfunction in a part of the nervous system called the endogenous opioid system. Below is a look at how this system works and how this system can malfunction with exposure to opioid drugs.
The endogenous opioid system
The endogenous opioid system helps regulate many physiologic processes, including how the body experiences pain, pleasure, reward, resilience to stress, and self-regulation of behaviors.
Like many systems in the body, the endogenous opioid system is intricate and complex. For a basic understanding, two key things to know are opioid peptides and opioid receptors. Peptides are substances produced by nerve cells. Made up of small strings of molecules, peptides attach to receptors, which are proteins found on or inside cells.
When a peptide attaches to a receptor, it causes a reaction by that cell. To use the endogenous opioid system as an example:
- Endorphins are a type of opioid peptide that are released by the brain in response to various events. Different types of endorphins are released in response to things like exercise, pain, and stress.
- Endorphins attach to opioid receptors (most commonly mu-opioid receptors, or MORs) found on nerve cells. As endorphins attach to these receptors, they stimulate a response by the nervous system.
- Endorphins released in response to pain will attach to MORs and dull the pain signals traveling through the nervous system.
Interactions between different types of peptides and receptors cause different responses by the nervous system.
The endogenous opioid system and OUD
Opioids are drugs that act on the endogenous opioid system. Opioids can refer to prescription pain medications as well as illegal drugs. Like the naturally occurring peptides described above, these drugs relieve pain and induce feelings of euphoria.
It cannot be emphasized enough that opioid drugs have a powerful effect on the brain. Repeated activation of opioid receptors by opioid drugs causes changes in how these receptors work. Imaging studies have found changes in brain structure and composition among people who have OUD. These changes contribute to the intense cravings for opioids a person with OUD will experience. This can occur even with short-term use of opioid drugs.
Treatment for OUD
If you or someone you know is living with OUD, it’s helpful to understand what is happening in the brain and nervous system when a person has OUD. Despite the persistent stigma and misinformation about OUD, this is not a disease that can be treated with willpower. It requires medical treatment.
Opioid receptors are a therapeutic target for medications for opioid use disorder (MOUD). There are medications that can effectively reduce opioid cravings, manage withdrawal symptoms, and help a person recover from OUD and maintain recovery. Counseling and therapy are also essential parts of a treatment plan.
However, treatment for OUD is different for everyone. For specific questions about treatment options for OUD for yourself or a loved one, your best source of information will be a healthcare provider.
