When you start naltrexone depends entirely on what you’re taking it for. For alcohol use disorder, you can typically begin right away with no waiting period. For opioid use disorder, you need to be completely opioid-free for a minimum of 7 to 10 days before your first dose. Starting too early after opioid use can trigger sudden, severe withdrawal, so getting the timing right is critical.
Starting Naltrexone for Alcohol Use Disorder
If you’re taking naltrexone to reduce drinking, there’s no mandatory waiting period. Your prescriber can start you at the standard dose of 50 mg once daily as soon as the prescription is filled. You don’t need to stop drinking before starting, though some people choose to. Naltrexone works by blocking the brain’s reward response to alcohol, which over time reduces cravings and makes drinking feel less rewarding.
Some prescribers use a different approach called the Sinclair Method, where you take naltrexone only on days you plan to drink, about one hour before your first drink. The idea is that by consistently blocking the pleasurable effects of alcohol at the moment of consumption, your brain gradually unlearns the association between drinking and reward. This targeted timing approach may be particularly helpful for binge drinkers.
Starting Naltrexone for Opioid Use Disorder
This is where timing becomes much more precise and the stakes are higher. The FDA-approved prescribing information specifies a minimum of 7 to 10 days opioid-free for people previously dependent on short-acting opioids like heroin, oxycodone, or hydrocodone. If you’ve been on longer-acting opioids like methadone or buprenorphine, you may need to wait at least two weeks, because these drugs clear from your system more slowly.
The reason for this waiting period is straightforward. Naltrexone is an opioid blocker. It binds to the same receptors in the brain that opioids attach to, but instead of activating them, it blocks them. If any opioid molecules are still sitting on those receptors when you take naltrexone, the medication forcibly displaces them. This causes what’s called precipitated withdrawal: a sudden, intense version of opioid withdrawal that hits within minutes rather than developing gradually over hours or days. Symptoms can include severe cramping, vomiting, diarrhea, anxiety, and agitation. It’s one of the most common reasons people abandon naltrexone treatment before it even begins.
How Your Provider Confirms You’re Ready
Simply counting days since your last opioid use isn’t always reliable. People metabolize drugs at different rates, and long-acting opioids can linger in the body longer than expected. To reduce the risk of precipitated withdrawal, many providers use a naloxone challenge test before prescribing naltrexone.
Naloxone is a short-acting opioid blocker (the same drug in Narcan). Your provider injects a small dose into your arm and watches for withdrawal signs over about 20 minutes. If nothing happens, a second slightly larger dose is given, followed by another 20-minute observation. Because naloxone wears off quickly, any withdrawal it triggers is brief and manageable. If you pass the challenge with no withdrawal symptoms, your provider knows it’s safe to start naltrexone. If the test triggers withdrawal, you’ll need to wait longer before trying again.
The First Dose and What to Expect
For opioid use disorder, providers often start with a half dose of 25 mg on the first day. If you tolerate that without any withdrawal symptoms, you move up to the full 50 mg dose the following day. This cautious approach catches any lingering opioid sensitivity that the challenge test might have missed. For alcohol use disorder, most people start directly at 50 mg.
Common side effects in the first few days include nausea, headache, dizziness, and fatigue. These typically fade within a week or two as your body adjusts. Taking naltrexone with food can help with nausea.
Oral Tablets vs. Monthly Injection
Naltrexone comes in two forms: a daily 50 mg tablet and a monthly injection (sold as Vivitrol). You do not need to take oral naltrexone first before getting the injection. Clinical trials for the injectable form did not require oral naltrexone before the first shot. That said, some providers prefer to start with a few days of oral naltrexone to confirm you tolerate the medication, since you can stop a pill but can’t remove an injection once it’s given. This is a practical preference, not a requirement.
The same opioid-free waiting period applies to both forms. Whether you’re starting tablets or the injection, you need 7 to 10 days free of short-acting opioids and up to two weeks free of methadone or buprenorphine.
Carrying Medical Alert Information
Once you start naltrexone, it’s important to carry identification noting that you’re on the medication. A medical alert bracelet, necklace, or wallet card all work. The reason: if you’re in an accident or need emergency surgery, standard opioid pain medications won’t work while naltrexone is active in your system. Emergency doctors need to know this so they can use alternative pain management strategies. Keep emergency contact information for your prescriber on you as well, so hospital staff can coordinate your care if needed.
For the oral form, naltrexone’s blocking effect lasts roughly 24 to 72 hours after your last dose, depending on how long you’ve been taking it. For the injection, the blocking effect persists for about a month. This is worth keeping in mind not just for emergencies but for any planned medical procedures that might involve opioid pain relief.