Naltrexone is a prescription medication primarily used to treat alcohol use disorder and opioid use disorder. It works by blocking opioid receptors in the brain, which reduces cravings for alcohol and prevents the euphoric effects of opioid drugs. It’s also used off-label at much lower doses for certain chronic pain and autoimmune conditions, and it plays a role in one FDA-approved weight loss medication.
How Naltrexone Works
Naltrexone blocks what’s called the mu-opioid receptor, the same receptor that opioid drugs like heroin and oxycodone activate to produce a high. Your body also has its own natural opioids (endorphins) that attach to these receptors, and these endorphins are part of the reason alcohol and opioids feel rewarding. By sitting on the receptor and preventing anything from activating it, naltrexone essentially mutes the brain’s reward response to these substances.
It also has weaker blocking effects on two other types of opioid receptors (kappa and delta), but the mu receptor is the primary target. The result is that drinking alcohol or using opioids while on naltrexone produces far less pleasure than it normally would, which over time weakens the learned association between the substance and the reward.
Treating Alcohol Use Disorder
For people trying to reduce or stop drinking, naltrexone works by dampening the pleasurable buzz that alcohol produces. This doesn’t make alcohol taste different or cause you to feel sick if you drink. Instead, the experience of drinking becomes less reinforcing. Over time, this helps reduce cravings and makes it easier to cut back or maintain sobriety.
In clinical trials, the most common side effects among people taking naltrexone for alcohol use disorder were nausea (about 10% of patients) and headache (about 7%). These side effects tend to be mild and often improve after the first few weeks.
Treating Opioid Use Disorder
For opioid use disorder, naltrexone serves a different but related purpose. Because it occupies opioid receptors without activating them, it completely blocks the effects of opioid drugs. If you take heroin, fentanyl, or prescription painkillers while on naltrexone, you won’t feel the high. This removes the incentive to use and provides a safety net during recovery.
There’s one critical requirement before starting: you must be completely free of all opioids first. The typical waiting period is 7 to 10 days after your last use of short-acting opioids (like heroin or oxycodone) and 10 to 14 days for long-acting opioids (like methadone or buprenorphine). Starting naltrexone too soon forces opioids off the receptors all at once, triggering sudden and severe withdrawal symptoms.
Pill vs. Monthly Injection
Naltrexone comes in two forms. The oral tablet is typically taken once daily at 50 mg. The injectable version, sold as Vivitrol, is a 380 mg dose given as a shot in the buttock muscle once every four weeks by a healthcare provider. After injection, the medication slowly releases from a polymer over the course of the month.
The injection has a practical advantage for many people in recovery: you can’t forget to take it or decide to skip it. Once injected, it stays in your system for the full month and can’t be removed. Blood levels peak within the first few days, then gradually taper over the following weeks, with measurable levels lasting beyond 30 days.
Weight Management
Naltrexone is one half of an FDA-approved weight loss combination that pairs it with bupropion (an antidepressant). The two drugs work together on appetite-regulating brain cells in a way that neither achieves alone. Bupropion stimulates neurons that produce signals telling your body to eat less. But those same neurons also release endorphins that circle back and dampen their own activity, a kind of built-in off switch. Naltrexone blocks that off switch, keeping the appetite-suppressing signal going longer and stronger.
In lab studies, each drug alone increased the firing rate of these appetite neurons to about 3 to 4 times per second. Together, they pushed it to roughly 11 times per second. Patients in clinical trials also reported that controlling their eating felt significantly easier while on the combination.
Low-Dose Naltrexone for Chronic Conditions
At much smaller doses, typically 1 to 6 mg per day compared to the standard 50 mg, naltrexone has gained attention for off-label use in autoimmune and chronic pain conditions. This approach, often called low-dose naltrexone or LDN, is thought to work through a different mechanism than the standard dose. At these tiny amounts, naltrexone may briefly block opioid receptors just enough to prompt the body to increase its own endorphin production and reduce inflammation.
The conditions most studied with LDN include fibromyalgia, where doses around 4.5 mg per day have been tested, as well as Sjögren’s syndrome, dermatomyositis, scleroderma, and rheumatoid arthritis. Dosing in these studies typically ranges from 1.5 to 4.5 mg per day, often starting low and gradually increasing. Results have been most encouraging for fibromyalgia pain and for severe itching associated with dermatomyositis and scleroderma. LDN remains off-label for all of these conditions, meaning it’s not FDA-approved for them, and the evidence base is still limited to smaller studies.
Naltrexone vs. Naloxone
These two medications are frequently confused because they have similar names and both block opioid receptors. The key difference is duration and purpose. Naloxone is a short-acting emergency drug used to reverse opioid overdoses. It wears off relatively quickly, which is why someone who receives naloxone during an overdose still needs monitoring for at least four hours, since the opioid can outlast the naloxone. Naltrexone is a long-acting maintenance medication taken daily or monthly to support ongoing recovery. It’s not used in emergencies.
Important Safety Considerations
Because naltrexone blocks opioid receptors, it also blocks opioid painkillers. If you’re on naltrexone and need pain management for a surgery or injury, standard doses of medications like codeine, morphine, or oxycodone simply won’t work. Non-opioid pain relief (like anti-inflammatory drugs or nerve blocks) is the first-line option in these situations. If opioids are absolutely necessary, higher-than-normal doses can be used, but only in a hospital setting with respiratory monitoring because of the risk of breathing problems. If surgery is planned, you’d typically stop the oral tablet at least 3 to 7 days beforehand.
Naltrexone is not prescribed to anyone currently using opioids, undergoing opioid withdrawal, or on opioid maintenance therapy like methadone or buprenorphine. It’s also not started when someone is expected to need opioid pain medication within the next week.
Liver health is monitored before and during treatment. Doctors typically check liver enzyme levels before prescribing naltrexone and periodically during use. Elevations in liver enzymes are possible, though clinically significant liver injury appears uncommon at standard doses. The concern is greater in people who already have liver disease, which is relevant since many people with alcohol use disorder have some degree of liver damage.