Naltrexone works by blocking opioid receptors in the brain, particularly the mu-opioid receptor. This single mechanism produces two distinct therapeutic effects: it prevents opioids from producing a high, and it reduces the rewarding sensation of drinking alcohol. The drug is FDA-approved for both opioid use disorder and alcohol use disorder, and a growing body of research supports low-dose use for inflammatory conditions.
Blocking Opioid Receptors
Naltrexone is a competitive antagonist, meaning it physically occupies opioid receptors without activating them. Think of it as a key that fits into the lock but won’t turn. While it sits in that lock, the actual opioid molecules (whether heroin, fentanyl, or prescription painkillers) can’t get in. The result: someone taking naltrexone who then uses an opioid will feel little to no effect from it. This eliminates both the euphoria and the physiological dependence that drive opioid addiction.
Naltrexone primarily targets the mu-opioid receptor, the one most responsible for the “high” from opioids. It also blocks kappa and delta opioid receptors, though more weakly. Once naltrexone is absorbed, the body converts much of it into an active metabolite called 6-beta-naltrexol, which continues blocking those same receptors. This metabolite actually lasts longer than naltrexone itself, with its blocking activity taking roughly 340 minutes to drop by half compared to 80 minutes for the parent drug. That extended action helps maintain consistent receptor coverage throughout the day.
How It Reduces Alcohol Cravings
Naltrexone’s effect on alcohol works through a less obvious pathway. When someone with alcohol dependence drinks, their brain releases beta-endorphin, the body’s own opioid. That endorphin triggers a cascade: it stimulates dopamine release in the brain’s reward center (the nucleus accumbens), both directly and indirectly by suppressing neurons that normally keep dopamine in check. This dopamine surge is what makes alcohol feel pleasurable and reinforces the desire to keep drinking.
Naltrexone interrupts this chain at the opioid receptor level. By blocking the receptors where beta-endorphin would normally land, it prevents that downstream dopamine surge. Alcohol still affects the brain in other ways (you still feel intoxicated), but the intense reward signal is dampened. Over time, this makes drinking feel less satisfying. Clinical trials have found that people on naltrexone average roughly 3 to 4.5 fewer heavy drinking days per month compared to placebo, a modest but meaningful reduction that, for many people, represents the difference between a destructive pattern and a manageable one.
This is why naltrexone for alcohol works best when combined with counseling or behavioral strategies. It doesn’t eliminate the urge to drink entirely. It quiets the neurological reward loop enough to give other coping tools room to work.
Oral Tablets vs. Monthly Injection
Naltrexone comes in two forms: a daily 50 mg oral tablet and a once-monthly 380 mg injection (sold as Vivitrol). They deliver the same drug but differ significantly in how the body processes it.
The oral tablet passes through the liver before reaching the bloodstream, a process called first-pass metabolism. The liver converts a large portion of the drug into 6-beta-naltrexol before it ever circulates. This isn’t necessarily a problem since the metabolite is active, but it does mean the ratio of naltrexone to its metabolite looks quite different from the injectable form.
The injection bypasses the liver initially. After a shot in the gluteal muscle, naltrexone plasma levels show an initial spike around 2 hours, then a second, higher peak at 2 to 3 days. From about day 14 onward, levels slowly taper but remain measurable for over a month. Total drug exposure from a single injection is 3 to 4 times higher than taking 50 mg daily by mouth for 28 days. The injection also produces significantly less of the 6-beta-naltrexol metabolite, since it avoids that first pass through the liver.
The practical advantage of the injection is adherence. Taking a pill every day requires daily motivation, and skipping doses reopens the window for opioid or alcohol relapse. A monthly shot removes that variable entirely.
The Opioid-Free Requirement
Starting naltrexone while opioids are still in your system can trigger precipitated withdrawal, an abrupt, intensified version of opioid withdrawal that hits within minutes. Because naltrexone rapidly displaces opioids from receptors, it essentially forces the brain into withdrawal all at once rather than gradually.
To avoid this, prescribing guidelines require a minimum of 7 to 10 days completely opioid-free before the first dose of naltrexone. For long-acting opioids like methadone, the required gap may be longer. This waiting period is one of the biggest practical barriers to naltrexone treatment for opioid use disorder, since it means completing detoxification first, a period when relapse risk is high.
Low-Dose Naltrexone and Inflammation
At standard doses (50 mg), naltrexone saturates opioid receptors. At very low doses, typically 1 to 4.5 mg, it appears to do something different. Low-dose naltrexone (LDN) interacts with a receptor on immune cells in the brain called Toll-like receptor 4 (TLR4), which is found on microglial cells. Microglia act as the brain’s immune system, and when chronically activated, they drive neuroinflammation linked to conditions like fibromyalgia, multiple sclerosis, and chronic pain syndromes.
Research on microglial cells shows that naltrexone can shift these immune cells from a highly activated, inflammatory state to a quieter, more balanced one. In lab studies, naltrexone reprogrammed the cells’ metabolism, moving them away from the rapid-energy inflammatory mode and toward a calmer oxidative metabolism driven by mitochondrial adaptation. It also modulated a key cellular pathway (mTOR/S6K) that governs how microglia behave immunologically, doing so in a dose-dependent manner.
LDN remains off-label for these inflammatory conditions, and the clinical evidence is still catching up to the biological rationale. But the mechanistic picture is becoming clearer: at low doses, naltrexone acts less as an opioid blocker and more as an immune modulator.
Safety and Liver Concerns
Naltrexone once carried an FDA black box warning for liver toxicity, which created lasting caution among prescribers. The FDA removed that warning in 2013 after finding no confirmed cases of liver failure attributable to naltrexone and minimal evidence that it worsens liver disease. A commonly cited threshold of liver enzymes at 5 times the upper limit of normal as a contraindication actually originated from old clinical trial exclusion criteria, not from specific evidence of harm at that level.
The most common side effects at standard doses are nausea, headache, dizziness, and fatigue, which tend to be worst in the first few days and often improve. At the injection site, Vivitrol can cause pain, hardness, or swelling that sometimes lasts several weeks. Serious injection site reactions, including tissue death, are rare but documented.
One critical safety consideration: naltrexone lowers opioid tolerance. If someone stops taking it and then uses opioids at the dose they previously tolerated, the risk of fatal overdose is significantly higher because their body has lost its tolerance during the period of receptor blockade.