Low-Dose Naltrexone Side Effects: Rare but Serious Adverse Events

What the labels cover, and what they do not

Naltrexone is a pure opioid antagonist that blocks opioid effects by competitive binding at opioid receptors [1]. The approved products are a 50 mg oral tablet, first sold as Revia (now discontinued, with generics still approved), for alcohol dependence and for blockade of the effects of exogenously administered opioids, and Vivitrol, a 380 mg deep gluteal injection every four weeks for alcohol dependence in patients able to abstain before starting and for prevention of relapse to opioid dependence after detoxification [1][6][2].

Low-dose naltrexone (LDN) is naltrexone at roughly 0.5 to 6 mg a day, most often 4.5 mg, used off-label and made mainly by compounding pharmacies [4]. No LDN product or LDN indication has FDA approval [4][5].

Neither the current generic 50 mg tablet label nor the Vivitrol label has a boxed warning [1][2]. Both labels discuss liver injury, precipitated withdrawal, depression and suicidality in their warnings sections [1][2]. The boxed warning sometimes attached to naltrexone in online discussion belongs to Contrave, a naltrexone-plus-bupropion weight-management product, where it covers suicidal thoughts and behaviors from the bupropion component [7].

Precipitated opioid withdrawal

When an opioid antagonist is given to someone physically dependent on opioids, the resulting withdrawal can be severe enough to require hospitalization, unlike ordinary withdrawal after stopping an opioid [1]. Symptoms have usually appeared within five minutes of ingestion and lasted up to 48 hours; reported cases include confusion, somnolence, visual hallucinations and fluid losses needing intravenous fluids, and postmarketing cases have needed hospital admission and sometimes intensive care [1]. It has also happened in alcohol-dependent patients whose prescriber did not know about their opioid use [1].

Before starting naltrexone tablets, the label recommends at least 7 to 10 opioid-free days after short-acting opioids and warns that withdrawal risk may persist for up to 2 weeks when switching from buprenorphine or methadone [1]. Vivitrol's label recommends at least 7 to 10 opioid-free days before an injection [2]. The labels list as contraindications receiving opioid analgesics, being currently dependent on opioids (including on methadone or buprenorphine maintenance), acute opioid withdrawal, and a positive urine screen for opioids or a failed naloxone challenge [1][2].

The contraindication is written without a dose exception, and the tablet label notes that a naloxone challenge test is not completely reliable [1]. At 4.5 mg, a fraction of the 50 mg tablet dose, the labeling still carves out no low-dose exemption [1].

Lowered opioid tolerance and overdose

The labels warn that patients may respond to lower opioid doses than they used to, so a previously tolerated amount can cause life-threatening intoxication after treatment is discontinued, at the end of a Vivitrol dosing interval, or after a missed dose; fatal overdoses have been reported [1][2]. The blockade itself is surmountable, and trying to push through it with large amounts of opioid can also be fatal [1][2]. Both labels say to strongly consider recommending or prescribing naloxone or nalmefene, and the Vivitrol label says to raise it at the initial injection and at every subsequent one [1][2].

Liver injury: what the labels actually say

The labels list hepatotoxicity in the warnings section rather than in a box. Cases of hepatitis and clinically significant liver dysfunction were seen during clinical development and after marketing, and both labels say to stop the product for symptoms or signs of acute hepatitis [1][2]. The dose relationship appears in the tablet label: placebo-controlled studies using up to 300 mg a day, five times the recommended dose, showed naltrexone causes hepatocellular injury in a substantial proportion of patients exposed at those higher doses [1]. In short-term controlled trials in alcohol dependence, AST elevations occurred in 1.5% of patients on Vivitrol, 1.5% on oral naltrexone and 0.9% on placebo [1][2].

LDN doses sit far below that range, but the labels were written for 50 mg tablets and a 380 mg injection and do not measure liver risk at LDN doses [1][2].

Depression and suicidal thoughts

The tablet label lists depression, suicide, attempted suicide and suicidal ideation among events reported after marketing, states that no causal relationship has been demonstrated, and says patients treated for alcohol or opioid dependence should be monitored [1]. In the label's own comparison table, new-onset depression ranged from 0 to 15% on naltrexone versus 0 to 17% on placebo, and suicide attempt or ideation from 0 to 1% versus 0 to 3% [1]. In the Vivitrol alcohol-dependence trial, depression was reported by 8% of 205 patients on 380 mg against 4% of 214 on placebo [2].

Autoimmune conditions

LDN is used off-label in inflammatory and autoimmune conditions partly on the theory that it modulates microglial activity, a mechanism a 2026 narrative review calls largely theoretical and speculative [4]. In an 8-week multiple sclerosis pilot that enrolled 80 people, 60 of whom completed it, LDN was well tolerated and serious adverse events did not occur [8].

What the LDN trials reported

Pooled data from four randomized fibromyalgia trials covering 222 patients found no significant difference in serious adverse events between LDN and placebo, while vivid dreams and nausea were more common on LDN [3]. The 12-month INNOVA trial of 4.5 mg in 98 women reported adverse events in 68.8% of the LDN group (n=48) and 72% of the placebo group (n=50), with no treatment-related serious events [9]. The 31-woman crossover trial reported no serious side effects [10]. A 2026 narrative review that screened 105 studies describes LDN as low-risk but unproven, notes that most evidence comes from small studies prone to publication bias, and reports that LDN trials most often describe insomnia, vivid dreams, nausea and dizziness [4].

Hypersensitivity and injection-only reactions

Both labels list hypersensitivity to naltrexone, or to a product's other components, as a contraindication [1][2]. The Vivitrol label records urticaria, angioedema and anaphylaxis, eosinophilic pneumonia, and injection site reactions that in some cases needed surgical debridement or left significant scarring, mostly in women [2]. Severe injection-site reactions are specific to Vivitrol.

The compounded product itself

Compounded drugs are not FDA-approved, and FDA states that it does not verify their safety, effectiveness or quality before they are marketed; poor compounding practice can produce contamination or a product with too much or too little active ingredient [5]. Naltrexone is not on FDA's list of bulk drug substances that may present significant safety risks [11], and the 2026 review notes that inconsistencies between compounded formulations add uncertainty [4].

Interactions the labels flag

The tablet label states that studies of interactions between naltrexone and non-opioid drugs have not been performed, so caution is advised with any concomitant drug [1]. Opioid-containing cough, cold and antidiarrheal medicines may not provide their intended benefit while taking naltrexone [1][2]. In an emergency needing opioid analgesia the amount required may be greater than usual, with deeper and more prolonged respiratory depression [1][2]. Disulfiram is not ordinarily combined with naltrexone because both can harm the liver, and lethargy and somnolence have been reported with thioridazine [1].

Pregnancy, kidney and liver impairment

The 50 mg tablet label carries the older Pregnancy Category C designation, with increased early fetal loss in rats and rabbits at high multiples of the human dose and no adequate well-controlled studies in pregnant women [1]. The Vivitrol label states that naltrexone and its major metabolite 6-beta-naltrexol are present in human milk, and recommends caution in patients with moderate to severe renal impairment [2]. A 2026 review notes that safety data for LDN in pregnancy and in liver or kidney impairment are sparse [4].

How often do rare events happen on LDN?

Adverse-event reports cannot establish how often an event happens or whether naltrexone caused it. FDA's public dashboard notes that duplicate and incomplete reports exist and that a report does not establish that the drug caused the event [12]. The tablet label's own summary is narrower than much of the discussion suggests: apart from dose-related hepatotoxicity and precipitated withdrawal in people who are not opioid-free, the available evidence does not incriminate naltrexone, used at any dose, as a cause of any other serious adverse reaction [1].