Naltrexone on its own is not a weight-loss drug, and its own trials say so. It is approved for alcohol dependence and to block opioids. It reaches weight-loss searches because it is one half of Contrave, and because some clinics prescribe it at low doses off-label. This page reports what the label and the trials record, dated, and where the evidence runs out.
Read on 12 November 2026 from the naltrexone hydrochloride tablet label (SpecGx, effective 4 September 2026, via the FDA's openFDA label service), Drugs@FDA, PubMed and ClinicalTrials.gov.
What the label is for
The tablet label gives two uses: alcohol dependence, and blocking the effects of opioids in people who have been through detoxification. It adds a sentence that is unusual for a label: naltrexone "has not been shown to provide any therapeutic benefit except as part of an appropriate plan of management for the addictions." Weight management is not mentioned as a use.
Drugs@FDA lists single-ingredient naltrexone tablets at 25 mg, 50 mg and 100 mg, all prescription-only. There is also a monthly injection (Vivitrol), approved for alcohol dependence and to prevent relapse to opioid dependence.
What the label says about weight
Weight appears twice in the label, and neither entry supports weight loss as a use.
- In the side-effect list, "increased appetite, weight loss, weight gain" sit together among events reported by fewer than 1% of people in the clinical studies. Loss of appetite is listed among events under 10%.
- In the laboratory section, the label describes a placebo-controlled study in people with obesity at 300 mg a day, about six times the dose used for opioid blockade: 19% of the naltrexone group (5 of 26) developed raised liver enzymes, against none of 24 on placebo. The label's warnings section covers hepatitis and liver dysfunction separately, and the dosing section notes a possibly higher risk of liver injury with single doses above 50 mg.
In other words, the one obesity study the label mentions appears there as a safety finding, not as evidence of effect.
The obesity trials of naltrexone alone
The idea came from animal studies, in which drugs that block opioid receptors reduced how much animals ate. The human trials that followed were small and short:
| Trial | Population | Length | Naltrexone | Result against placebo |
|---|---|---|---|---|
| Atkinson et al., Clin Pharmacol Ther 1985 | 60 adults with obesity | 8 weeks | 50 or 100 mg/day | No significant weight loss in either group. Women alone lost 1.7 kg (a subgroup, significant); men did not lose weight. Six people had abnormal liver tests |
| Malcolm et al., Int J Obes 1985 | 41 adults (27 women, 14 men), 30–100% overweight | 10 weeks | 200 mg/day | −1.8 kg vs −1.5 kg, no significant difference. Three people had liver enzymes twice the normal level |
Both papers suggested higher doses or longer trials, with liver monitoring. No larger trial of naltrexone alone for obesity followed. Two small trials later tested it against weight gain in specific groups:
- Antipsychotic weight gain. In Tek et al., 2014, 24 overweight women with schizophrenia or schizoaffective disorder took 25 mg a day or placebo for 8 weeks: −3.40 kg against +1.37 kg. The authors called it a pilot and said a larger study was under way.
- Weight gain after quitting smoking. In Toll et al., 2010, 172 smokers worried about weight took 25 mg or placebo for 27 weeks. Weight gain at 26 weeks was 6.8 lb against 9.7 lb, not a significant difference, and quit rates were slightly lower on naltrexone (22% against 27%, also not significant).
Naltrexone inside Contrave
The approved weight use of naltrexone is in Contrave, approved on 10 September 2014. Each extended-release tablet contains 8 mg of naltrexone and 90 mg of bupropion — far less naltrexone than the 50 to 200 mg tested in the 1980s trials. These are product strengths, quoted from the label.
The case for the pairing came from trials that included a naltrexone-alone arm. A 2009 trial in 419 adults with uncomplicated obesity (Greenway et al., JCEM) compared the combination with each drug alone and with placebo over 24 weeks. Placebo-subtracted weight loss for the combinations was −3.5% to −4.7%, and the combinations beat naltrexone alone in every comparison the trial reported. A companion paper reported that people on naltrexone alone or placebo diverged from those on the combination from week 16. Neither abstract gives a separate percentage for naltrexone alone, and this page does not estimate one.
The bupropion half has its own page: Wellbutrin and weight loss.
Low-dose naltrexone
"Low-dose naltrexone", often shortened to LDN, is sold by some clinics and telehealth services for weight, inflammation and other conditions. The record for weight is thin, and it can be counted:
- It is not an approved product. No approved naltrexone tablet comes below 25 mg, so smaller doses are made by compounding pharmacies.
- ClinicalTrials.gov, searched on 12 November 2026 for low-dose naltrexone and obesity, returned 2 studies. One tests Contrave in a rural clinic; the other compares ways of maintaining weight loss after GLP-1 treatment. Neither tests low-dose naltrexone alone for weight.
- PubMed, searched the same day for "low dose naltrexone" with weight or obesity, returned 22 records. By title and abstract, the only human weight trial among them is the 25 mg smoking trial above, which found no significant difference. The rest are cancer, pain, depression and animal studies, protocols and case reports.
So no published placebo-controlled trial has shown that low-dose naltrexone causes weight loss. That is an absence in the record, not proof that it does nothing; the question has simply not been tested in a trial anyone has published.
Beside the approved weight medicines
For scale, with the populations named: in Contrave's COR-I trial (56 weeks, adults with obesity or overweight), weight fell 5.4% against 1.3% on placebo. In their own trials in adults with obesity and no diabetes, semaglutide 2.4 mg gave −14.9% against −2.4% on placebo over 68 weeks (STEP 1), and tirzepatide 15 mg gave −20.9% against −3.1% over 72 weeks (SURMOUNT-1). No trial compares naltrexone alone with any of them, and the 1985 trials ran for 8 to 10 weeks, so the figures cannot be set against each other as a ranking. The other non-GLP-1 options are on GLP-1 alternatives and the tablet register is on weight-loss pills.
Safety the label prints
The tablet label's warnings include:
- Opioid withdrawal and overdose. Naltrexone can bring on sudden withdrawal in anyone using opioids, and people who stop it can be more sensitive to opioids than before, which raises overdose risk.
- Liver. Cases of hepatitis and liver dysfunction were seen in the development programme and after marketing; most people with raised enzymes had other possible causes, such as alcoholic liver disease or hepatitis B or C.
- Depression and suicidal thoughts. Depression, suicide, attempted suicide and suicidal ideation have been reported after marketing in people treated for opioid dependence.
That is a summary of the warnings as printed, not a full list. A medicine that blocks opioid painkillers is not interchangeable with a weight medicine, and whether it suits a particular person is a decision for the prescribing clinician.
What is not established
- No regulator has approved naltrexone alone for weight, so no label gives a weight endpoint for it.
- The obesity trials of naltrexone alone are from 1985, small (41 and 60 people) and short (8 and 10 weeks). No modern trial has repeated them.
- The 2009 trials report that the combination beat naltrexone alone, but their abstracts do not give a weight figure for naltrexone alone.
- No published placebo-controlled trial has tested low-dose naltrexone for weight loss.
This is where a reader should stop and ask a clinician. Whether naltrexone, alone or in Contrave, suits one person depends on opioid use, liver health and mood history, and that is a medical decision this page does not and cannot make.
Sources
- Naltrexone hydrochloride tablets USP, prescribing information, SpecGx LLC (ANDA 076264), effective 4 September 2026 — openFDA drug label record, set ID 06ff2d5a-e62b-4fa4-bbdb-01938535bc65, read 12 November 2026: indications, warnings, adverse reactions, laboratory tests, dosage.
- Drugs@FDA product records for naltrexone hydrochloride (openFDA drugsfda), read 12 November 2026.
- Atkinson RL et al., Clinical Pharmacology & Therapeutics 1985, PMID 4042525. Malcolm R et al., International Journal of Obesity 1985, PMID 3908352. Greenway FL et al., Journal of Clinical Endocrinology & Metabolism 2009, PMID 19846734. Greenway FL et al., Obesity 2009, PMID 18997675. Tek C et al., Journal of Clinical Psychopharmacology 2014, PMID 25102328. Toll BA et al., Drug and Alcohol Dependence 2010, PMID 20542391.
- ClinicalTrials.gov API v2, searches for naltrexone + obesity (58 studies) and low dose naltrexone + obesity (2 studies: NCT03431831, NCT07832084), 12 November 2026. PubMed search "low dose naltrexone" AND (weight OR obesity), 22 records, 12 November 2026.
- Wilding JPH et al., NEJM 2021, PMID 33567185 (STEP 1). Jastreboff AM et al., NEJM 2022, PMID 35658024 (SURMOUNT-1). COR-I figures as reported on this site's Contrave page, label read 3 November 2026.