A standard 50 mg oral dose of naltrexone blocks the effects of opioids for up to 24 hours. The injectable form, Vivitrol, maintains its blockade for approximately 30 days. The exact duration depends on which formulation you’re taking, the dose, and your individual metabolism.
Oral Naltrexone: 24 to 72 Hours
At the standard 50 mg daily dose, oral naltrexone blocks the effects of opioids for as long as 24 hours. FDA clinical data show that this dose neutralizes the effects of 25 mg of intravenously administered heroin for that full window. Doubling the dose to 100 mg extends the blockade to about 48 hours, and tripling it to 150 mg pushes it to roughly 72 hours. These higher doses are sometimes used in supervised dosing schedules where a patient takes the medication every two or three days instead of daily.
Naltrexone itself has a relatively short half-life of about 4 hours, which might make that 24-hour blockade seem surprising. The explanation lies in what your body converts the drug into. Your liver breaks naltrexone down into a byproduct that has a half-life of 12 to 14 hours, roughly three times longer than the parent drug. This byproduct is a weaker opioid blocker than naltrexone itself, but it builds up to much higher concentrations in your blood, especially with daily dosing. That accumulation is a key reason the blockade lasts well beyond what naltrexone’s own half-life would suggest.
Injectable Naltrexone: About 30 Days
The extended-release injectable form is given as a 380 mg intramuscular injection once every four weeks. It maintains a therapeutic blood level of naltrexone over that entire period, blocking the effects of opioids and opioid-based medications for at least 30 days. That includes not just heroin and prescription painkillers but also opioid-based cough suppressants and antidiarrheal medications.
Because the drug is embedded in a slow-dissolving matrix injected into muscle tissue, it releases gradually rather than all at once. This eliminates the daily decision of whether to take a pill, which is one reason the injectable version is often preferred for people in early recovery when motivation can fluctuate day to day.
What Happens During an Emergency
One practical concern for anyone on naltrexone is what happens if you need emergency pain relief. If you’re injured or need surgery while the blockade is active, standard opioid painkillers won’t work normally. Emergency protocols call for non-opioid pain management first: regional nerve blocks, local anesthesia, or non-opioid pain medications.
If opioid-based pain relief is absolutely necessary, it can only be administered safely in a closely monitored medical setting with staff trained in advanced airway management and resuscitation. The risk is that much higher doses of opioids would be needed to overcome the blockade, which creates a narrow margin between effective pain control and dangerous respiratory depression. Carrying a medical alert card or wearing a medical ID bracelet that identifies you as a naltrexone patient helps emergency teams make the right call quickly.
Increased Overdose Risk After Stopping
When naltrexone wears off, whether after missing oral doses or as an injection fades, your opioid receptors don’t simply return to their previous state. While the drug is active, your brain compensates for the blockade by becoming more sensitive to opioids. Animal research shows this heightened sensitivity develops during treatment and takes roughly six days to return to baseline levels after stopping.
This matters enormously for overdose risk. If you use the same amount of opioids you tolerated before starting naltrexone, your body may now respond far more intensely to that dose. The window right after the blockade fades is one of the highest-risk periods for fatal overdose. Your previous tolerance is gone, and your receptors may be temporarily more reactive than they were before you ever started treatment.
The Required Opioid-Free Period Before Starting
Naltrexone can’t be started while opioids are still in your system. If it is, it will instantly displace opioids from your receptors and trigger precipitated withdrawal, a sudden, intense version of withdrawal that can take a full day or longer to resolve. Unlike the gradual onset of normal withdrawal, precipitated withdrawal hits within minutes and is significantly more severe.
To avoid this, you need to be completely opioid-free for a minimum of 7 to 10 days before your first dose of either oral or injectable naltrexone. If you’re switching from methadone, which lingers in the body much longer than most opioids, a full medically managed withdrawal must be completed before that 7 to 10 day clock even starts. This waiting period is one of the biggest practical barriers to starting naltrexone, since it requires getting through the hardest stretch of withdrawal before the medication’s protective effects kick in.
Factors That Affect Duration
The blockade timelines above represent averages from clinical studies, but individual variation exists. Liver function plays a significant role since naltrexone is processed almost entirely by the liver. People with faster metabolisms may clear the drug sooner, while those with impaired liver function may experience a longer blockade. Body weight, age, and overall health also influence how quickly the drug is metabolized.
For the injectable form, injection technique and site matter too. The medication must be delivered into the gluteal muscle, not into fatty tissue. A shallow injection that ends up in subcutaneous fat rather than muscle can alter how the drug is released, potentially shortening or making the blockade less consistent. This is why injections should always be given by a trained provider using the correct needle length.