Naltrexone is not an opioid. It is an opioid antagonist, meaning it blocks opioid receptors rather than activating them. While opioids like morphine or heroin bind to receptors in the brain and produce euphoria, pain relief, and sedation, naltrexone latches onto those same receptors and prevents anything from happening. It produces no high, no pain relief, and no physical dependence.
Why the Confusion Exists
Naltrexone gets lumped in with opioids because it operates in the same biological territory. It targets the same receptors, particularly the mu-opioid receptor, which is the primary target of drugs like heroin, fentanyl, and prescription painkillers. The critical difference is what happens after it binds. An opioid activates the receptor and triggers a cascade of effects. Naltrexone occupies the receptor without activating it, essentially locking the door so opioids can’t get in.
Adding to the confusion, naltrexone is chemically derived from a similar molecular backbone as some opioids. Its structure is close enough to fit into opioid receptors with high precision, but the shape is just different enough to block rather than stimulate them. Think of it as a key that fits into the lock but won’t turn.
What Naltrexone Actually Does
Naltrexone is FDA-approved to treat both opioid use disorder and alcohol use disorder. It was first approved in 1984 for heroin addiction, then later for alcohol dependence. It works differently for each condition, but the core mechanism is the same: blocking the brain’s opioid receptors.
For opioid addiction, the logic is straightforward. If someone takes heroin or a prescription painkiller while on naltrexone, the drug can’t reach its target. The person feels little to no effect, which removes the reward that drives repeated use.
For alcohol, the connection is less obvious but well established. When you drink, your brain releases its own natural opioids (endorphins), which contribute to the pleasurable buzz alcohol produces. Naltrexone blocks those endorphins from reaching their receptors, dulling alcohol’s rewarding effects. Drinking feels less satisfying, which over time can reduce cravings and help people cut back. Naltrexone also appears to influence the body’s stress-hormone system in ways that further suppress the drive to drink.
It Has No Abuse Potential
Because naltrexone blocks rather than activates opioid receptors, it cannot produce a high. There is no euphoria, no sedation, and no risk of becoming addicted to the medication itself. It is not a controlled substance and does not require the special prescribing restrictions that medications like methadone or buprenorphine do. You won’t experience cravings for naltrexone or withdrawal symptoms if you stop taking it abruptly.
This is one of the key distinctions between naltrexone and other medications used to treat opioid addiction. Methadone is a full opioid agonist (it activates opioid receptors and can itself be addictive). Buprenorphine is a partial agonist (it mildly activates receptors). Naltrexone sits on the opposite end of the spectrum as a pure blocker.
How It’s Taken
Naltrexone comes in two forms. The oral version is a daily tablet. The injectable form, sold under the brand name Vivitrol, is a 380 mg intramuscular injection given once every four weeks. The injectable version bypasses the liver’s initial processing, so the total monthly dose is considerably lower than what you’d take orally over the same period (roughly 1,500 mg in daily tablets over a month versus the single 380 mg injection).
The injectable option is often preferred for opioid use disorder because it eliminates the daily decision to take a pill. Missing doses can be dangerous for reasons covered below.
The Opioid-Free Requirement
One important safety consideration: you must be completely free of opioids for at least 7 to 10 days before starting naltrexone. If any opioids are still attached to your receptors when naltrexone arrives, it will rip them off and immediately trigger severe withdrawal symptoms. This is called precipitated withdrawal, and it can be intensely uncomfortable, coming on within minutes rather than the gradual onset of normal withdrawal.
This requirement can make starting naltrexone challenging for people with opioid dependence, since it means completing detoxification first. Your provider will typically confirm you’re opioid-free before giving the first dose.
Reduced Tolerance After Treatment
Perhaps the most critical safety issue with naltrexone is what happens after you stop taking it. During treatment, your body’s tolerance to opioids drops significantly because the receptors haven’t been exposed to opioid stimulation. If you relapse after discontinuing naltrexone, the dose you used to take before treatment can now be enough to cause a fatal overdose.
SAMHSA warns that even lower doses than what a person previously used can cause life-threatening consequences after a period of naltrexone-supported abstinence. The blocking effect fades gradually after the last dose, and during that transition period, sensitivity to opioids is dangerously high. Anyone taking naltrexone for opioid use disorder should make sure the people closest to them understand this risk.