Naltrexone vs Naloxone: What's the Real Difference?

If you've ever Googled one of these drugs and landed on information about the other, you're in good company. Naltrexone and naloxone share a prefix, a mechanism, and almost nothing else. One works in under five minutes and wears off in an hour. The other sticks around for a full day and is FDA approved for alcohol use disorder. We'll walk through what each one does so you know exactly which is which.

TLDR:

• Naltrexone is the one used for alcohol; naloxone has no FDA approval for drinking
• Naloxone acts in 2-5 minutes and wears off in 30-90 minutes (rescue only)
• Naltrexone lasts 24-72 hours, blocking the reward loop that keeps drinking automatic
• First-week side effects like nausea usually fade; weight gain occurs in under 1% of cases
• Onir prescribes naltrexone 50 mg online at $99/month across all 50 states

What Are Opioid Antagonists?

Opioid receptors are proteins scattered across the brain and spinal cord that respond to both the body's own endorphins and external opioids like heroin or oxycodone. When activated, they produce pain relief, euphoria, and sedation. An opioid antagonist binds to these same receptors without activating them, locking the door so opioids (or endorphins) can't get through.

Both naltrexone and naloxone are opioid receptor antagonists, competing with opioids for binding sites at the mu, kappa, and delta receptors. That shared mechanism is where the similarity ends. Naloxone works fast and wears off quickly, making it the go-to for reversing an overdose in minutes. Naltrexone sticks around much longer and is prescribed for ongoing treatment of alcohol use disorder and opioid use disorder. The two drugs get confused constantly, despite filling completely different roles in clinical practice.

Naltrexone: What It Is and How It Works

Naltrexone is an opioid antagonist that has been FDA approved for alcohol use disorder for over 30 years. It comes in two forms: an oral 50 mg tablet (brand name ReVia) and a 380 mg monthly injection (Vivitrol).

When you drink, your brain releases endorphins that bind to opioid receptors and create a rewarding feeling, reinforcing the habit over time. Naltrexone blocks those receptors, so the endorphin surge from alcohol has nowhere to land. Drinking still happens if you choose it, but the reinforcement loop weakens. Over weeks and months, this process, sometimes called pharmacological extinction, reduces cravings and makes alcohol feel less automatic.

The oral tablet has a half-life of roughly 4 hours, but its active metabolite, 6-beta-naltrexol, lingers for around 12 hours, which is what makes once-daily dosing practical. Naltrexone also carries a separate FDA approval for opioid use disorder, where it serves a maintenance role after detox by keeping opioid receptors blocked, following a naltrexone dosing guide tailored to each condition.

Naloxone: What It Is and How It Works

Naloxone is also an opioid antagonist, but it was built for speed. Administered as a nasal spray (sold as Narcan), intramuscular injection, or auto-injector, it binds to opioid receptors within 2 to 5 minutes and can reverse the respiratory depression that makes an opioid overdose fatal. Since 2023, Narcan nasal spray has been available over the counter in the U.S., putting it in the hands of bystanders and first responders without a prescription.

The tradeoff for that rapid onset is a short duration of action, typically 30 to 90 minutes. Because many opioids, especially fentanyl and extended-release formulations, outlast naloxone in the body, a single dose can wear off while the opioid is still active. Overdose protocols often call for repeated doses and continued monitoring until emergency medical help arrives.

This short window is why naloxone fills an emergency role and nothing more. It clears the system too quickly to serve as ongoing treatment for alcohol use disorder or opioid maintenance, which is where naltrexone's longer duration becomes relevant.

Naltrexone vs Naloxone: Key Pharmacological Differences

Both drugs block opioid receptors competitively, but their pharmacokinetic profiles send them in opposite directions clinically.

The short version: naloxone is a rescue drug, naltrexone is a treatment drug. Naloxone's brief window can pull someone out of an overdose but can't keep receptors blocked long enough to reduce cravings. Naltrexone's extended duration is what makes daily or monthly dosing feasible for conditions like alcohol use disorder or opioid use disorder.

Naltrexone for Alcohol Use Disorder

To answer the title question directly: naltrexone is the one used for alcohol. Naloxone has no FDA approval for treating alcohol use disorder and no clinical role in reducing drinking.

Naltrexone works by blocking the opioid receptors that endorphins activate when you drink, dulling the rewarding buzz that keeps the habit looped in place. Two prescribing approaches are common. Daily dosing means taking a 50 mg tablet every morning regardless of whether you plan to drink. The Sinclair Method takes a targeted route: you take the tablet about an hour before drinking so the receptor blockade is active during the session itself, a timing strategy covered in more depth in our guide on when to take naltrexone.

The treatment gap is staggering. About 28 million Americans aged 12 and older live with alcohol use disorder, yet fewer than 10% receive any treatment, and only about 2.5% receive medication. Naltrexone remains one of only three FDA-approved medications for the condition, alongside acamprosate and disulfiram.

Common Side Effects of Naltrexone

The first week on naltrexone is when side effects are most noticeable. Nausea tops the list, followed by headache, fatigue, dizziness, and reduced appetite. For most people, these fade within a week or two as the body adjusts.

Taking the tablet with food cuts down on nausea considerably, and many clinicians suggest starting at 25 mg for the first several days before moving up to the full 50 mg. If side effects stick around or feel strong, messaging your clinician before changing anything is the right call.

Weight gain comes up often in online threads, but according to the drug maker's prescribing information, it was reported in fewer than 1 out of 100 people. Naltrexone is actually more likely to cause a smaller appetite and mild weight loss. When people do gain weight after starting, the cause is usually recovery itself (better nutrition, sugar cravings replacing alcohol) and not the medication.

Long term, the side effect profile stays mild. There are no well-documented cumulative toxicities at standard doses, though periodic liver safety monitoring is sometimes recommended as a precaution.

Where Naloxone Fits: Role in Suboxone, Narcan, and Combination Medications

Naloxone shows up in several products, each time playing a different supporting role.

In Narcan, naloxone is the sole active ingredient, delivered as a nasal spray to reverse opioid overdoses. In Suboxone, it plays a subtler part. Suboxone combines buprenorphine (a partial opioid agonist that eases withdrawal and cravings) with naloxone. The naloxone component is there to deter misuse: taken as a sublingual tablet or film, buprenorphine absorbs well while naloxone barely reaches the bloodstream. If someone dissolves and injects the combination, though, the naloxone becomes fully active and triggers withdrawal, discouraging that route of administration.

This makes Suboxone fundamentally different from naltrexone. Suboxone partially activates opioid receptors while guarding against injection misuse. Naltrexone is a pure antagonist with no opioid activity at all. Methadone sits on the opposite end as a full opioid agonist, dispensed through licensed clinics for opioid dependence. All three treat opioid use disorder, but through entirely different mechanisms, and only naltrexone carries an FDA approval for alcohol.

Naltrexone vs Acamprosate and Disulfiram for Alcohol

Three FDA-approved medications exist for alcohol use disorder, and each one works through a distinct pathway.

Naltrexone blocks opioid receptors, weakening the reward signal that alcohol triggers. Acamprosate (brand name Campral) modulates glutamate and GABA activity to calm the neurological restlessness that often follows prolonged abstinence. Disulfiram (Antabuse) interferes with alcohol metabolism, causing nausea, flushing, and headache if you drink on it. The deterrent is physical discomfort, not craving reduction.

According to a meta-analysis of 64 trials, acamprosate was slightly more effective at maintaining complete abstinence, while naltrexone was slightly more effective at reducing heavy drinking days. If the goal is drinking less, naltrexone tends to be the better fit.

Disulfiram requires strong motivation and often supervised dosing, since skipping a pill removes the consequence entirely. Naltrexone and acamprosate are generally considered first-line options, with disulfiram reserved for patients who respond well to an aversion-based approach, and our guide to alternatives to naltrexone covers how to weigh these options.

Safety Warnings and Drug Interactions

The most serious risk with naltrexone is starting it too soon after opioid use. If you've taken any opioid (heroin, fentanyl, oxycodone, codeine, or others) within the past 7 to 10 days, naltrexone can trigger precipitated withdrawal, a reaction that can be severe and rapid. Naloxone carries the same risk in opioid-dependent individuals, though its short duration means symptoms resolve faster.

Naltrexone is contraindicated for anyone with acute hepatitis or liver failure, and periodic liver function tests are sometimes recommended. Pregnancy and breastfeeding are also contraindications.

Kratom acts on the same opioid receptors naltrexone blocks, so combining them is not recommended, and it's one of several naltrexone drug interactions worth knowing. If you use kratom, bring it up with your clinician before starting. The same goes for any other supplement, prescription, or substance you're taking.

Mnemonic for Remembering Naltrexone vs Naloxone

A simple memory trick circulates through pharmacology study groups and holds up well:

"NalTRExone is for TREatment. NalOXone keeps you OXygenated."

Treatment versus oxygen. Long-term care versus emergency rescue. That single pairing captures the core split between the two drugs, and it's usually enough to get through a board exam question or a fast clinical decision without second-guessing which is which, though for a deeper look at how naltrexone helps you drink less, it helps to understand the craving mechanism itself.

How Onir Prescribes Naltrexone for Alcohol Online

At Onir, we prescribe naltrexone online using FDA-approved generic naltrexone hydrochloride 50 mg tablets through a telehealth model available in all 50 states. A licensed clinician reviews each intake through a secure, asynchronous text-based chat, usually within 24 hours.

The process is straightforward: a free 60-second quiz, then a $19.99 clinician review (refunded in full if you're not approved). If prescribed, the cost is $99 a month, billed as $297 every 90 days, with medication and clinician care included. No insurance paperwork, no in-person visits, no pharmacy trips, and because naltrexone is not a controlled substance, medication ships directly to you in plain, unmarked packaging. You can cancel anytime from your dashboard, and the care team is reachable by message throughout your program if questions come up, including what happens when drinking on naltrexone.

Final Thoughts on How Naltrexone and Naloxone Compare

Same receptor targets, very different timelines. Naloxone buys minutes during an overdose. Naltrexone works across weeks and months to change how your brain responds to alcohol or opioids. If you're weighing your options for cutting back on drinking, naltrexone is the medication built for that job.

FAQ

Naltrexone vs naloxone for alcohol — which one actually treats drinking?

Naltrexone is the one used for alcohol. It blocks the opioid receptors that endorphins activate when you drink, weakening the reward loop over time. Naloxone has no FDA approval for alcohol use disorder and no clinical role in reducing drinking — it exists to reverse opioid overdoses.

Can I get naltrexone through a telehealth service that doesn't bill insurance?

Yes. Onir prescribes FDA-approved naltrexone 50 mg tablets entirely online, available in all 50 states, on a self-pay basis with no insurance paperwork. The cost is $99 a month (billed as $297 every 90 days), medication and clinician care included — and because Onir doesn't bill insurance, nothing appears in your insurance record.

How long do naltrexone side effects last in the first week?

Nausea, headache, fatigue, and dizziness are most noticeable during the first several days and typically fade within one to two weeks. Taking the tablet with food reduces nausea, and many clinicians suggest starting at 25 mg before moving up to the full 50 mg dose.

What happens if you take naltrexone and Suboxone together?

You should not combine them. Suboxone contains buprenorphine, a partial opioid agonist, and naltrexone is a pure opioid antagonist — taking naltrexone while buprenorphine is still in your system can trigger precipitated withdrawal, which is severe and immediate. A clinician needs to confirm you've been off all opioids for 7 to 10 days before starting naltrexone.

How is naltrexone different from acamprosate for alcohol use disorder?

Naltrexone blocks the reward signal alcohol triggers through opioid receptors, while acamprosate calms the glutamate and GABA imbalance that causes restlessness after prolonged abstinence. A meta-analysis of 64 trials found acamprosate slightly better at maintaining complete abstinence, while naltrexone was slightly better at reducing heavy drinking days — so if your goal is drinking less, naltrexone tends to be the stronger fit.

Is naltrexone an opioid?

No. Naltrexone is an opioid antagonist, meaning it blocks opioid receptors without activating them. It produces no euphoria, no sedation, and no physical dependence — it is non-addictive and non-habit-forming.

Are naloxone and naltrexone both antagonists, and if so why are they used so differently?

Yes, both are opioid receptor antagonists that block mu, kappa, and delta receptors. The difference is pharmacokinetics: naloxone acts in 2–5 minutes and wears off within 90 minutes, making it a rescue drug for overdoses, while naltrexone lasts 24–72 hours, making it suitable for ongoing treatment of alcohol use disorder or opioid use disorder.

What's a good mnemonic for remembering naloxone vs naltrexone on a USMLE exam?

The most popular one is: "NalTRExone is for TREatment, NalOXone keeps you OXygenated." Treatment vs. oxygen maps directly to the clinical split — long-term craving reduction vs. emergency overdose reversal.

Naltrexone vs methadone vs Suboxone — how do I make sense of all three?

They work through completely different mechanisms. Methadone is a full opioid agonist dispensed at regulated clinics. Suboxone combines buprenorphine (a partial opioid agonist) with naloxone to deter misuse. Naltrexone is a pure antagonist with zero opioid activity — and it's the only one of the three with an FDA approval for alcohol use disorder.

Does naltrexone cause weight gain?

Weight gain is reported in fewer than 1% of clinical trial participants. Naltrexone is actually more likely to reduce appetite slightly. When people do gain weight after starting, the typical cause is recovery-related changes — better nutrition and sugar cravings replacing alcohol calories — not the medication itself.

Can I build a drinking-reduction plan with naltrexone without going fully sober?

Yes. Naltrexone supports whatever goal you set — drinking less on weeknights, taking longer breaks, or stopping entirely. Two common approaches exist: daily dosing (50 mg every morning) and the Sinclair Method (taking the tablet about an hour before you plan to drink). Your clinician can help you choose the approach that fits your goal.

Should I take naltrexone every day or only before drinking?

Both approaches are FDA-supported. Daily dosing means one 50 mg tablet each morning regardless of plans. The Sinclair Method targets doses to about an hour before drinking sessions, so the receptor blockade is active when alcohol hits. Each approach has tradeoffs, and a clinician can tailor the plan to your habits and goals.

What are the long-term side effects of naltrexone?

At standard doses, the long-term side effect profile stays mild with no well-documented cumulative toxicities. Most side effects (nausea, headache, fatigue) are front-loaded in the first week or two and taper off. Periodic liver function monitoring is sometimes recommended as a precaution, and anyone with acute hepatitis or liver failure should not take the medication.

How does low dose naltrexone differ from the standard 50 mg tablet for alcohol?

Low dose naltrexone (typically 1–5 mg) is used off-label for conditions unrelated to alcohol, such as chronic pain or autoimmune issues, and operates through a different proposed mechanism. The FDA-approved dose for alcohol use disorder is 50 mg. Onir prescribes the standard 50 mg tablet, which is the dose backed by decades of clinical evidence for reducing drinking.

Why is naloxone included in Suboxone if it's a rescue drug?

In Suboxone, naloxone acts as a misuse deterrent. Taken as a sublingual film or tablet, buprenorphine absorbs well while naloxone barely reaches the bloodstream. But if someone dissolves and injects the combination, naloxone becomes fully active and triggers withdrawal — discouraging that route of administration. It's a design safeguard, not a treatment ingredient.