Taking too much naltrexone primarily puts your liver at risk and can cause intense gastrointestinal distress, confusion, and mood changes. The standard dose is 50 mg per day, and clinical trials testing doses up to 300 mg daily (six times the norm) found liver damage in a substantial proportion of participants. A single extra pill is unlikely to cause a medical emergency, but the risks climb meaningfully as the dose increases.
The Standard Dose and Where Problems Begin
Naltrexone is prescribed at 50 mg once daily for both alcohol use disorder and opioid dependence. Treatment for opioid dependence typically starts even lower, at 25 mg, before stepping up. The FDA label carries a specific warning: there may be a higher risk of liver injury with single doses above 50 mg. In studies where participants received up to 300 mg per day, liver cell damage occurred frequently enough that it became one of the drug’s most prominent safety concerns.
Liver Damage Is the Biggest Concern
Naltrexone’s most serious risk at high doses is hepatotoxicity, meaning direct injury to liver cells. At standard doses of 50 to 100 mg daily, modest increases in liver enzyme levels show up in some patients. At 300 mg daily, the picture changes significantly. In one study of 26 patients receiving that dose, nearly one in five developed elevated liver enzymes. Across broader trials, enzyme elevations above three times the normal upper limit occurred in roughly 1% of patients, and up to 30% showed some degree of increase, an effect that appeared to be partially dose-dependent.
The encouraging finding is that these elevations consistently resolved once the drug was stopped. But the signs of liver stress are worth knowing: unusual tiredness, loss of appetite, pain in the upper right side of your abdomen lasting more than a few days, dark urine, light-colored stools, or yellowing of the skin or eyes. Any of these after taking a higher-than-prescribed dose signals that your liver is struggling to process the drug.
Immediate Physical Symptoms
Beyond liver effects, taking too much naltrexone can cause severe nausea and vomiting, diarrhea, abdominal cramps, and dizziness. At standard doses, nausea and headache are already among the most common side effects. Higher amounts intensify these considerably. Confusion, hallucinations (seeing or hearing things that aren’t there), and blurred vision have also been reported as serious side effects that warrant immediate medical attention.
In extreme cases, a naltrexone overdose can lead to seizures, difficulty breathing, or loss of consciousness.
Mood and Mental Health Effects
Naltrexone works by blocking opioid receptors in the brain, which also play a role in mood regulation. Even a single standard 50 mg dose has been shown to cause dysphoria (a general sense of unease or unhappiness) in healthy volunteers with no history of drug use. At higher doses, this effect can intensify into depression or significant anxiety.
An uncontrolled cohort study of 81 patients on naltrexone for opioid dependence found higher-than-expected rates of both overdose (on other substances) and suicide, raising concerns about the drug’s psychiatric effects. People taking naltrexone tended to experience increased anxiety soon after starting treatment. Taking more than prescribed could amplify these mood disruptions unpredictably, particularly in someone already managing depression or anxiety.
A Special Risk: Precipitated Withdrawal
If you take naltrexone while opioids are still in your system, the risk isn’t just a standard overdose reaction. It’s precipitated withdrawal, a sudden, severe withdrawal syndrome that can begin within minutes. Naltrexone rips opioids off the brain’s receptors all at once, triggering a cascade of symptoms: nausea, vomiting, sweating, rapid heartbeat, tremors, muscle aches, abdominal cramps, anxiety, irritability, fever, dilated pupils, and intense cravings. Many patients describe it as the worst withdrawal they’ve ever experienced.
This is why patients are required to be completely opioid-free, including tramadol, for at least 7 to 10 days before starting naltrexone. People transitioning from longer-acting opioid medications like methadone or buprenorphine may need up to 14 days. Taking a higher dose of naltrexone while any opioids remain in your body makes this reaction more likely and more severe.
How Long the Effects Last
Naltrexone is processed by the liver into an active metabolite that continues blocking opioid receptors even as the parent drug clears. After a standard dose, the drug’s opioid-blocking effects persist for roughly 24 to 72 hours depending on the amount taken. A larger dose extends that window, meaning both the therapeutic blockade and any side effects will last longer. Liver enzyme elevations in clinical trials took days to weeks to normalize after the drug was stopped, though they consistently did resolve.
Who Faces the Greatest Risk
People with pre-existing liver conditions face the most danger from excess naltrexone. The drug is contraindicated in anyone with acute hepatitis or liver failure. People with alcohol use disorder or a history of injection drug use often already have compromised liver function, which means their liver has less capacity to handle a higher dose safely. Anyone with HIV or hepatitis C co-infection is also at elevated risk, as case reports have documented sharp liver enzyme spikes in these populations even at standard doses.
There is no specific antidote for naltrexone overdose. Treatment in a medical setting is supportive, focused on managing symptoms like nausea, dehydration, and vital sign changes while the drug clears your system. If you’ve taken significantly more than your prescribed dose and develop any combination of severe vomiting, confusion, abdominal pain, or signs of liver distress, that warrants emergency evaluation.