Your body makes several opioid-like signals on its own. The same receptor family that relieves pain in one circuit can slow or stop breathing in another, which is why understanding how these medicines differ can be lifesaving.
Part 16 of the Brain Chemistry series. New here? Start with Your Brain Is Not a Gas Tank, the short orientation that explains the four questions every article in this series answers.
What to know
- Your body makes several different opioid-like peptides, not one single “endorphin” chemical, from different starting proteins and cells.
- Four receptors matter most: mu, delta, kappa, and NOP. Turning the mu receptor on can relieve pain in one circuit and slow breathing in another.
- Tolerance, physical dependence, withdrawal, and addiction are four different things. Dependence is not addiction, and it can develop during ordinary, appropriate medical care.
- Buprenorphine only partly activates the mu receptor, but that does not make overdose impossible, especially combined with alcohol, benzodiazepines, or other sedating substances.
- Recognizing and responding to an opioid overdose can save a life: know the warning signs, give naloxone if you have it, and call 911 right away.
The short answer
Endogenous means made by the body. Your body makes several opioid-like peptide families: beta-endorphin, enkephalins, dynorphins, and a signal called nociceptin, each from a different starting protein, made in different cells across the hypothalamus, pituitary, striatum, mood circuits, spinal cord, brainstem, and body.
These peptides act on four related receptors: mu, delta, kappa, and NOP (short for MOR, DOR, KOR, and NOP). Turning one on usually quiets the cell it reaches. But quieting one cell does not always mean quieting a whole circuit: the quieted cell may itself have been holding another cell back, so silencing that brake lets the second cell fire more. This is called disinhibition, one reason opioid effects are harder to predict than “more signal, more calm.”
These systems help control pain, reward-based learning, stress, gut movement, hormone signals, and breathing. Endorphin is not another word for happiness or pleasure. More opioid receptor activity can ease pain in one circuit, and in another it can cause dangerously slowed or stopped breathing, called respiratory depression.
Medicines built around this system serve very different goals. Methadone fully activates the mu receptor. Buprenorphine only partly activates it. Naltrexone blocks opioid receptors entirely, to help prevent relapse. Naloxone also blocks them, but fast, specifically to reverse a suspected overdose. Four different plans, not four versions of the same fix.
Your body does not make one substance called “endorphins.” Four separate genes make four separate starting proteins that are then cut down into the active peptides: POMC becomes beta-endorphin plus other hormones, PENK becomes met-enkephalin and leu-enkephalin, PDYN becomes a group of dynorphins, and PNOC becomes nociceptin, also called orphanin FQ.
The finished peptides are stored in small packets inside nerve cells. Nerve activity releases them, they reach a nearby opioid receptor, and enzymes outside the cell break them down quickly. There is no single pump that clears them all the way some other brain chemicals are cleared.
All four receptors work the same basic way once turned on: they lower a signal inside the cell that tends to mean less signal release or less firing. But that single-cell effect does not predict the whole circuit. Mu receptor activity can quiet a cell that normally holds a reward-related dopamine cell in check, and with that brake released, the dopamine cell fires more. The same mu receptor activity on a pain nerve ending reduces pain signals directly, and in a breathing-control network it weakens the drive to breathe. One receptor family, three different outcomes, depending on which circuit it acts in.
Medicines are not simply extra amounts of your body’s own peptides. Fentanyl, methadone, buprenorphine, and other opioid drugs differ from each other, and from your own peptides, in strength, how well they enter the brain, how long they last, and how strongly they turn the receptor on.
Meet the four opioid receptors
Four receptors do nearly all of this work.1
| Receptor | What it does, where it matters most, and why you might care |
|---|---|
| MOR (mu) | What it does: Usually reduces cell firing. Where it matters most: Pain, reward, brainstem breathing control, gut, hormone, and body sensory pathways. Why you might care: Main target for pain relief and for dependence and dangerously slowed breathing. Methadone and many pain medicines turn it on fully; buprenorphine partly; naltrexone and naloxone block it. |
| KOR (kappa) | What it does: Usually reduces cell firing; often activated by dynorphins. Where it matters most: Stress, aversion, pain, and mood circuits. Why you might care: A major recent depression-drug target. Two KOR-blocking drugs went through large trials and both failed to show enough benefit. |
| DOR (delta) | What it does: Usually reduces cell firing; often activated by enkephalins. Where it matters most: Forebrain, sensory, pain, and body circuits. Why you might care: Studied for pain and emotion; no routine medicine selectively targets it. |
| NOP | What it does: Usually reduces cell firing; activated by nociceptin. Where it matters most: Cortex, mood circuits, hypothalamus, brainstem, and spinal cord. Why you might care: Involved in pain, reward, and stress research; no approved medicine selectively targets it. |
Partial agonist does not mean weak, or safe
Buprenorphine binds tightly to the mu receptor, but only partly turns it on, even at a dose that occupies nearly all of it. That partial activation caps some effects compared with a full agonist like methadone or fentanyl, and it can push a full agonist off the receptor if one is already there, which can trigger withdrawal in a person dependent on it.
This matters for a safety reason that deserves to be stated plainly: partial agonism does not remove overdose risk. Breathing can still slow dangerously, especially when buprenorphine is combined with alcohol, benzodiazepines, or other sedating substances. “Partial” describes the receptor action, not a guarantee of safety.
Tolerance, dependence, withdrawal, and addiction are not the same thing
These four words get used interchangeably, but they mean different things, and mixing them up leads to real harm, including unfair judgment of people in appropriate medical treatment.
Tolerance means repeated exposure produces less of a certain effect over time. It does not grow at the same rate for every effect: tolerance to pain relief, euphoria, constipation, sleepiness, and slowed breathing develops at different speeds, part of why overdose risk can rise even in someone who feels the drug “doesn’t do much anymore.”
Physical dependence is a body change that can lead to withdrawal if the drug is reduced, stopped, or pushed off the receptor by a blocker. It can develop during entirely appropriate, well-managed medical care, and it is not, by itself, anything to be ashamed of.
Withdrawal is the set of symptoms that follows: distress, pain, diarrhea, vomiting, sweating, restlessness, and poor sleep, with timing that depends on the specific drug.
Addiction, including opioid use disorder (OUD), is different again. It involves compulsive use despite harm. Dependence alone is not addiction. A person can have both together, but a person can also be physically dependent on a medicine that is genuinely helping them without meeting the definition of addiction at all.
Relapse is a return to use after a period of reduced or no use. It carries a specific danger: tolerance can fall substantially during that time away, and overdose risk rises for the same reason after naltrexone, a receptor blocker, is stopped. A return to a previously “normal” amount, an unexpectedly strong product, or an attempt to override naltrexone’s blockade with more opioid can all be life-threatening. Naltrexone also requires a clinician-led, opioid-free transition before starting it, because beginning it while opioids are still in the body can trigger severe withdrawal.
Which symptoms need a call, and which need urgent help
Track and mention at your next visit: constipation, mild nausea, sleep changes, pain levels, cravings, mood, and daily function. Bring your full medicine and substance list. Physical dependence is worth discussing openly, without shame.
Call the prescriber or pharmacist promptly: increased sleepiness, repeated vomiting, hard-to-manage constipation, new faintness, or tooth problems from a medicine that dissolves in the mouth, a missed or interrupted OUD-medicine dose, a return to use, pregnancy, or starting a new benzodiazepine, alcohol, or sedative. Do not stop an OUD medicine on your own over an interaction concern; call first.
Treat a suspected overdose as an emergency, every time. The warning signs are: the person cannot be woken, their breathing is slow or absent, they are making choking or gurgling sounds, or their skin or lips look blue, gray, or pale. If you see any of these signs:
- Give naloxone right away if you have it available.
- Call 911.
- Stay with the person.
- Follow the instructions on the naloxone product and whatever the 911 operator tells you.14
- Be ready to give another dose. Naloxone can wear off before the opioid does, so symptoms can return, and more doses may be needed if the person does not respond or symptoms come back.
For a suspected medicine mix-up or exposure without severe symptoms, call U.S. Poison Control at 1-800-222-1222. Do not let that call delay a 911 call if there are any breathing problems, collapse, seizure, or inability to wake the person: call 911 first in those situations.
Call 911 for slow or stopped breathing, inability to wake someone, collapse, seizure, severe confusion, severe chest symptoms, or danger of any kind right now. For a suicidal, mental health, or substance use crisis, call or text 988. Naloxone is safe to give whenever an opioid overdose is suspected, even if you turn out to be wrong. It does not replace emergency care, and it will not reverse other drugs that might be involved in a mix.
What each medicine actually touches
| Medicine | What it does directly, used for, and main tradeoffs |
|---|---|
| Buprenorphine (alone or with naloxone) | What it does directly: Partially activates mu; also blocks kappa. Combination products add a separate naloxone blocker. Used for: Opioid use disorder treatment. Main tradeoffs: Breathing can still slow with sedating substances. Can trigger withdrawal in someone dependent on a full agonist. Schedule III. |
| Methadone | What it does directly: Fully activates mu. Used for: OUD care under federal access rules; some products treat pain. Main tradeoffs: Slowed breathing, drug buildup, interactions, QT prolongation, a heart rhythm risk. Schedule II. |
| Naltrexone (oral or long-acting shot) | What it does directly: Blocks opioid receptors, most strongly mu. Used for: OUD and alcohol use disorder care. Main tradeoffs: Can trigger severe withdrawal if opioids remain in the system; lowers tolerance, raising later overdose risk. Not controlled. |
| Naloxone | What it does directly: Rapidly blocks opioid receptors. Used for: Reversing a suspected overdose. Main tradeoffs: Can trigger withdrawal; can wear off before the opioid does. Some nasal products are sold without a prescription. Not controlled. |
| Nalmefene nasal spray | What it does directly: Rapidly blocks opioid receptors. Used for: Known or suspected overdose. Main tradeoffs: Can trigger withdrawal; other drugs in a mix can still cause harm. Prescription only, age 12 and up. Not controlled. |
| Morphine, oxycodone, fentanyl, and other opioid pain medicines | What it does directly: Fully activate mu, with drug-specific differences. Used for: Acute, cancer, comfort, surgical, or long-term pain, by product. Main tradeoffs: Misuse potential, dependence, constipation, sleepiness, interactions, fatal slowed breathing. Many Schedule II. |
| Lofexidine | What it does directly: Acts on a different receptor, not an opioid receptor. Used for: Easing some adult withdrawal symptoms. Main tradeoffs: Low blood pressure, slow heart rate, fainting, heart rhythm changes, sleepiness. Treats withdrawal, not OUD itself. |
Buprenorphine, methadone, and naltrexone are FDA-approved opioid use disorder medicines and are named in national addiction-medicine treatment guidance.234567 Federal controlled-substance scheduling for these medicines was checked against current DEA records.13
Two failed depression drugs, and one honest finding about treatment
Because dynorphin and the kappa receptor are tied to stress and aversion in animal research, two drug companies developed KOR-blocking medicines and tested them for major depression, hoping the animal effect would translate into a human antidepressant. Both failed. Aticaprant’s phase 3 depression program was stopped for too little benefit,9 though a separate, earlier-stage aticaprant schizophrenia study was still recruiting as of the most recent check.10 Navacaprant missed its main goal across a phase 2 study and three phase 3 studies,1112 and its depression development ended. Neither result means kappa receptor biology has no role in stress. A well-reasoned idea, tested carefully in large human trials, did not produce a working treatment.
A separate review pooling 30 group studies covering more than 370,000 people found that time spent in OUD medicine treatment, such as buprenorphine, methadone, or naltrexone, was associated with fewer deaths than time not in treatment or after leaving it.8 This was observational research, not a randomized trial, so other factors could be influencing the result. It still argues against the idea that medicine-based treatment is just trading one addiction for another: the data point toward treatment being associated with staying alive.
Can your body’s own opioids be measured?
Not in any way that is useful for diagnosis or treatment decisions.
Toxicology tests, commonly called drug tests, can detect certain opioids or their breakdown products for a limited window of time. A result does not measure your own opioid peptides, receptor activity, how severe an addiction is, the dose taken, or whether someone is impaired or being honest, and some substances are missed unless the right test is used.
Blood tests for beta-endorphin mostly reflect hormone activity outside the brain, shaped by stress, exercise, and sample handling. Spinal fluid sits closer to the brain but still pools many sources. PET scans can estimate available receptors or drug occupancy, but this is a research tool, not a diagnostic one. Gene tests may look at the mu receptor gene or genes affecting drug breakdown, but cannot show current opioid peptide activity.
No routine blood, urine, saliva, hair, spinal fluid, brain scan, or gene test can diagnose a global “low endorphin” state.
What to ask your prescriber
These are conversation starters, not instructions.
- Which symptom or daily problem is this medicine meant to target?
- Is it a full agonist, a partial agonist, or a blocker, and what does that mean for me?
- What improvement should we look for first, and by when?
- Which effects settle on their own, and which need a prompt call?
- What breathing, sleepiness, heart rhythm, or injection-site risks apply here?
- What are the risks of combining this with benzodiazepines, alcohol, or other sedatives?
- Should I keep naloxone on hand, and can people around me learn to use it?
- What should I do if I miss a dose, or treatment is interrupted?
- Could stopping suddenly cause withdrawal, relapse, or overdose risk?
Bottom line
The same receptor family that relieves pain can also stop someone’s breathing, which is why full agonists, partial agonists, and blockers do such different jobs. Dopamine works through a similarly branching system, and so does the body’s own cannabis-like system: one messenger family, several jobs, different circuits. Physical dependence is not addiction, a medicine response does not prove someone started with low endorphins, and your brain is not a gas tank to be topped up or drained. What matters most in practice: recognize overdose signs, know naloxone can wear off before the opioid does, and remember tolerance falls fast after time away from opioids, exactly when a return to use is most dangerous.
Frequently asked questions
Does opioid use disorder mean someone has low endorphins?
No. It is diagnosed from behavior, daily function, and harm, not a peptide level. A response to medicine does not prove a low level existed beforehand.
Is physical dependence the same as addiction?
No. Dependence is a body change that carries a withdrawal risk. Addiction involves compulsive use despite harm. Both deserve care, but they are different things.
Is buprenorphine just replacing one addiction with another?
No. It has a clear, well-understood receptor action and a well-supported role in treatment. It can cause physical dependence, but dependence is not uncontrolled, harmful use.
Does being a partial agonist mean buprenorphine cannot cause an overdose?
No. Breathing can still slow dangerously, especially with alcohol, benzodiazepines, or other sedating substances.
If buprenorphine and methadone both activate the mu receptor, why does naltrexone, a blocker, also help?
They serve different goals. Agonist treatment turns the receptor on in a controlled way; blocker treatment stops opioid effects entirely after a safe, clinician-led transition. The best fit depends on more than the mechanism alone.
Does keeping naloxone on hand encourage opioid use?
No. Naloxone reverses a life-threatening reaction. It does not create a high; keeping it available is a safety step, not an endorsement of harmful use.
Can a urine test prove addiction or impairment?
No. It can detect certain drugs or breakdown products within testing and timing limits. It cannot diagnose opioid use disorder, measure impairment, or explain why a substance was present.
Why is overdose risk especially high after abstinence or after naltrexone is stopped?
Tolerance falls during that time away. A return to a previously normal dose, an unexpectedly strong product, or an attempt to push through naltrexone’s blockade can stop breathing and can be fatal.
Related reading on NP FADY
- Your Brain Is Not a Gas Tank (start here)
- What Your Body’s Own Cannabis-Like System Does
- Why Two Opposite Medicines Both Target Dopamine
- Low-Dose Naltrexone (LDN) in Psychiatry: A Comprehensive Guide
References
1. IUPHAR/BPS opioid receptor record. Guide to Pharmacology. Accessed August 31, 2026. https://www.guidetopharmacology.org/GRAC/FamilyDisplayForward?familyId=50
2. FDA overview of medications for opioid use disorder. Accessed August 31, 2026. https://www.fda.gov/drugs/food-and-drug-administration-overdose-prevention-framework/information-about-medications-opioid-use-disorder-moud
3. ASAM national practice guideline. Accessed August 31, 2026. https://www.asam.org/quality-care/clinical-guidelines/national-practice-guideline
4. Buprenorphine and naloxone prescribing information. DailyMed. Accessed August 31, 2026. https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=4210afeb-474c-d842-d68e-af7e0021851a
5. Methadone prescribing information. DailyMed. Accessed August 31, 2026. https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=7ff89589-149c-4007-8144-5f60131b8c33
6. Extended-release naltrexone prescribing information. DailyMed. Accessed August 31, 2026. https://dailymed.nlm.nih.gov/dailymed/fda/fdaDrugXsl.cfm?setid=cd11c435-b0f0-4bb9-ae78-60f101f3703f&type=display
7. Lofexidine prescribing information. DailyMed. Accessed August 31, 2026. https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=2dcc8288-adbe-45c3-b7bd-4d274001332d
8. Ma and colleagues. Pooled analysis, 30 cohort studies, 370,611 participants, mortality and time in OUD medicine treatment. https://doi.org/10.1038/s41380-018-0094-5
9. Johnson & Johnson statement on the aticaprant phase 3 depression program, stopped 2025. Press release. Accessed August 31, 2026. https://www.jnj.com/media-center/press-releases/johnson-johnson-statement-on-ventura-program
10. Phase 1b aticaprant schizophrenia study, recruiting at cutoff. ClinicalTrials.gov, NCT07615426. Accessed August 31, 2026. https://clinicaltrials.gov/study/NCT07615426
11. Neumora Therapeutics releases on KOASTAL-1, KOASTAL-2, and KOASTAL-3 navacaprant studies and end of development, 2026. Press releases. Accessed August 31, 2026. KOASTAL-1: https://ir.neumoratx.com/news-releases/news-release-details/neumora-therapeutics-reports-data-koastal-1-study-navacaprant. KOASTAL-2 and KOASTAL-3: https://ir.neumoratx.com/news-releases/news-release-details/neumora-therapeutics-reports-data-phase-3-koastal-program-and/
12. Mathew and colleagues. Peer-reviewed phase 2 navacaprant trial. https://doi.org/10.1097/JCP.0000000000001967
13. DEA controlled-substance scheduling reference. Accessed August 31, 2026. https://www.deadiversion.usdoj.gov/schedules/schedules.html
14. CDC overdose prevention guidance and FDA naloxone guidance. Accessed August 31, 2026. CDC: https://www.cdc.gov/overdose-prevention/prevention/index.html. FDA: https://www.fda.gov/consumers/consumer-updates/access-naloxone-can-save-life-during-opioid-overdose
This article is general education. It is not a diagnosis or a treatment plan. Do not start, stop, or change a medicine because of something you read here. Review every prescription, supplement, and substance with your prescriber or pharmacist. If you or someone you know is in a mental health or substance use crisis, call or text 988. If someone is showing signs of an opioid overdose, give naloxone if available and call 911 immediately.
If you or someone you know is in crisis
- Call 911 or go to your nearest emergency room for any life-threatening emergency.
- 988 Suicide & Crisis Lifeline — call or text 988, available 24/7. En español: marque 988 y oprima 2. Veterans: 988 and press 1, or text 838255.
- Crisis Text Line — text HOME to 741741.
- The Trevor Project (crisis support for LGBTQ+ young people) — call 1-866-488-7386, or text START to 678-678.
- National Sexual Assault Hotline (RAINN) — call 1-800-656-HOPE (4673) or text HOPE to 64673; free, confidential, 24/7. Online chat at RAINN.org/hotline.
- National Domestic Violence Hotline — call 1-800-799-SAFE (7233) or text START to 88788; 24/7, help in 200+ languages. Online chat at TheHotline.org. If your phone or computer may be monitored, calling from a safer device is an option.
- Riverside County — Inland SoCal Crisis Helpline 951-686-HELP (4357), 24/7 (Inland SoCal United Way / 211+, in partnership with RUHS-BH); Community Access, Referral, Evaluation and Support (CARES) Line 800-499-3008, 24/7.
- San Bernardino County — Access Unit (Behavioral Health Helpline) 888-743-1478, 24/7; Mobile Crisis/CCRT 800-398-0018 (24/7, all ages) or text 909-420-0560. Arrowhead Regional Medical Center (ARMC) has a dedicated walk-in adolescent psychiatric ER (ages 13–17).
- Children under 13 — call 911 for immediate danger, contact your county's mobile crisis team (they respond to all ages), or go to the nearest pediatric emergency room.
- California Peer-Run Warm Line (non-crisis — someone to talk to) — call or text 1-855-600-WARM (9276); daytime and evening hours, not a 24/7 line.
- NP Fady (non-emergency) — for routine scheduling or questions, call (909) 707-6261. This line is not monitored for emergencies.