Opioid and fentanyl detox: the timing problem, the medications, and what comes after.
Opioid withdrawal is rarely fatal on its own, but it is severe enough to drive people back to use, and with fentanyl the return is often deadly. This page explains why fentanyl behaves differently from heroin or pills, how buprenorphine is started without making things worse, and why medication does not stop when detox ends.
Authored by the MLJ Clinical Team. Reviewed under board-certified psychiatric oversight. Last updated September 2026.
Key Takeaways- Heroin and short-acting pill withdrawal begins within about 8 to 12 hours, peaks at two to three days, and eases over a week; fentanyl's onset is often delayed and its course longer, because the drug is stored in fat and released slowly.
- That delay creates a specific hazard: starting buprenorphine too soon after fentanyl can trigger precipitated withdrawal, a sudden and severe worsening of symptoms.
- Severity is tracked with the Clinical Opiate Withdrawal Scale (COWS), and comfort medications such as clonidine, ondansetron, and loperamide manage symptoms while the induction is timed.
- The three FDA-approved medications for opioid use disorder are buprenorphine, methadone, and naltrexone; SAMHSA's TIP 63 is clear that they are treatment, not a crutch, and they continue into residential care and beyond.
- The most dangerous moment in opioid recovery is the return to use after tolerance has dropped; the medication plan exists in large part to make that moment survivable and less likely.
What opioid withdrawal is.
Opioids act on receptors that regulate pain, mood, breathing, gut motility, and the body's stress response. Used daily, they suppress the brain's own stress chemistry, and the brain compensates by turning that chemistry up. When the opioid is removed, the compensation is exposed. The noradrenaline system in particular fires hard, and the result is the picture most people know from film: sweating, gooseflesh, yawning, runny nose, dilated pupils, muscle and bone pain, cramping, vomiting, diarrhea, restless legs, insomnia, and an anxiety that feels bottomless. People describe it as the worst flu of their lives, with a dread underneath it.
Unlike alcohol and benzodiazepine withdrawal, opioid withdrawal in an otherwise healthy adult is very rarely fatal in itself. Deaths that do occur are usually from dehydration and electrolyte loss through vomiting and diarrhea in someone who cannot get care, or from an underlying heart condition. The real danger is different. Withdrawal is so aversive, and so reliably stopped by the drug, that most unsupervised attempts end in a return to use within days. And the return to use, after even a short period of abstinence, is when overdose deaths cluster, because tolerance falls faster than the habit does. Our page on what to do in the first 24 hours after a relapse exists for exactly that reason.
For heroin and short-acting prescription opioids such as oxycodone, the timeline is well established. Symptoms begin roughly 8 to 12 hours after the last dose, peak between 48 and 72 hours, and largely subside by day seven, with sleep and mood disturbances persisting for weeks. For long-acting opioids like methadone, onset is later, at 24 to 48 hours or more, and the course stretches across two weeks or longer. Fentanyl, which now dominates the illicit opioid supply in Los Angeles and nationally, does not fit neatly into either category, and that is the problem this page is mostly about.
Fentanyl's timing problem.
Fentanyl is a synthetic opioid roughly fifty times more potent than heroin, according to the National Institute on Drug Abuse. Given once, in a hospital, it is also short-acting. That is not the fentanyl a daily user is dealing with.
Fentanyl is highly lipophilic, meaning it dissolves readily into fat. With repeated daily use, it accumulates in fat tissue and is released back into the bloodstream slowly over days. Clinicians who work with fentanyl-dependent patients have documented, and increasingly published on, two consequences. First, withdrawal often begins later than the person expects: not at hour twelve but sometimes at day two or three. People arrive at detox feeling deceptively well and are hit hard on the third day. Second, the course is longer and the peak broader; fentanyl withdrawal can feel more like a long-acting opioid's course than a short-acting one's, and symptoms may extend well into the second week.
The third consequence is the one with the most direct clinical stakes. Buprenorphine, the medication most often used to treat opioid withdrawal and opioid use disorder, is a partial agonist with a very strong grip on the opioid receptor. If it is given while a full agonist is still substantially occupying those receptors, it displaces the full agonist and, because it activates the receptor less strongly, produces an abrupt drop in opioid effect. The person goes from mild withdrawal to severe withdrawal within an hour. This is precipitated withdrawal, and because fentanyl lingers in the body, the window in which it can happen is longer and harder to judge than it is with heroin. Managing that window safely is a large part of what a fentanyl detox is.
Measuring severity: the COWS scale.
The Clinical Opiate Withdrawal Scale, or COWS, is the standard tool for measuring where a person is in withdrawal at a given moment. A nurse or physician rates eleven items: resting pulse, sweating, restlessness, pupil size, bone or joint aches, runny nose or tearing, gastrointestinal upset, tremor, yawning, anxiety or irritability, and gooseflesh. The total runs from zero to 48. Scores of 5 to 12 are considered mild, 13 to 24 moderate, 25 to 36 moderately severe, and above 36 severe. It is repeated on a schedule throughout detox and forms the backbone of both comfort-medication decisions and the timing of buprenorphine.
| COWS score | Severity | What it tells the team |
|---|---|---|
| 5–12 | Mild | Withdrawal has begun; comfort medications; too early for standard buprenorphine induction |
| 13–24 | Moderate | Objective signs present; the range in which standard induction is typically considered |
| 25–36 | Moderately severe | Intensive symptom management; hydration and electrolytes monitored closely |
| Above 36 | Severe | Medical evaluation for complications; uncommon in a planned, supervised detox |
The scale's value lies in its objective items. Pupil size, pulse, sweating, and gooseflesh cannot be exaggerated or minimized, which matters because opioid withdrawal is so miserable that a person's own report is understandably urgent. The objective signs are also what tell a physician that enough of the full agonist has left the receptors to make buprenorphine safe. With heroin that judgment is fairly reliable. With fentanyl it is harder, and a COWS score alone is not enough; time since last use, the pattern of use, and how symptoms are trending across several scores all inform the decision.
Comfort medications and the first days.
While the timing for buprenorphine is being worked out, and for people who choose not to use an opioid-based medication at all, a set of non-opioid medications keeps withdrawal tolerable. Clonidine, and its newer relative lofexidine, which the FDA approved specifically for opioid withdrawal in 2018, act on the noradrenaline system that drives sweating, anxiety, cramping, and the racing heart. Ondansetron treats nausea and vomiting. Loperamide treats diarrhea. Non-steroidal anti-inflammatories address bone and muscle pain. Hydroxyzine or trazodone are used for anxiety and sleep. Intravenous or oral fluids and electrolytes replace what the gut is losing. None of these is a cure for withdrawal; together they take a COWS score down several points and turn an unbearable experience into a hard one.
The first days at MLJ, then, look like this. You arrive and are assessed: what you use, how much, when the last dose was, what else you take, and what has happened in previous attempts to stop. Bloodwork and a physical exam look for what opioid use conceals or causes, including infection, liver disease, and cardiac problems. A baseline COWS is taken. Comfort medications begin as symptoms appear. You are in your own room with nursing present around the clock, and someone is checking your score and your vital signs on a schedule that adapts to what the scores are doing.
If your history is fentanyl, we will be honest with you that the second and third days may be worse than the first, that the timing of buprenorphine will be set by your body rather than by the calendar, and that the alternative, rushing it, is worse. This is one of the places where a true one-to-one clinical ratio is not a luxury. Watching the trend across repeated scores on one person is how the decision is made well.
Buprenorphine induction and precipitated withdrawal.
Buprenorphine is the medication SAMHSA's Treatment Improvement Protocol 63 describes in most detail for office-based and residential settings, because it can be prescribed outside a specialized clinic and because it does two things at once: it relieves withdrawal, and it treats opioid use disorder over the months and years that follow. As a partial agonist it produces enough opioid effect to stop withdrawal and craving without producing the high, and its ceiling effect on breathing makes it far safer in overdose than full agonists. Combined with naloxone in the common sublingual formulations, it is also less attractive to misuse.
The standard induction waits until a person is in clear, objective withdrawal, typically a COWS score in the moderate range, so that enough of the full agonist has cleared for buprenorphine to improve things rather than precipitate them. The first dose is given, the person is observed, and the dose is adjusted over the first day or two to the level that holds withdrawal and craving. With heroin this usually happens twelve to twenty-four hours after the last use. With fentanyl the wait can be longer, and there is real uncertainty in the field about how long is long enough for any given person.
Because of that uncertainty, physicians have developed alternatives to the standard induction for fentanyl. One approach is to wait longer and use comfort medications more aggressively in the meantime. Another, often called low-dose or "micro" induction, starts buprenorphine at a very small dose while the person is still using or still has fentanyl on board and increases it gradually over several days, so that buprenorphine occupies receptors slowly enough that no precipitated withdrawal occurs. The evidence base for low-dose induction is still developing, but it has become common practice in settings that see a great deal of fentanyl. If precipitated withdrawal does occur, it is treated, it passes within hours, and it does not mean buprenorphine has failed; it means the timing was off, and the team adjusts.
Naltrexone, methadone, and choosing a path.
Buprenorphine is one of three FDA-approved medications for opioid use disorder. The second is methadone, a full agonist taken daily, which is highly effective and has the longest evidence base, but which under federal law can only be dispensed for opioid use disorder through a licensed opioid treatment program. MLJ is not an opioid treatment program, so methadone is not started here; if you arrive on methadone from an OTP, or if it is the right medication for you, we coordinate with a program rather than pretend the option does not exist.
The third is naltrexone, an opioid antagonist that blocks the receptor entirely. It produces no opioid effect, has no potential for misuse, and in its extended-release injectable form is given once a month. Its constraint is the opposite of buprenorphine's: naltrexone cannot be started until the person is fully free of opioids, because giving an antagonist to someone with opioids still on their receptors causes severe precipitated withdrawal. For heroin that means an interval of about a week after the last use; for fentanyl, given its slow release from tissue, the interval may need to be longer, and the physician will typically confirm readiness with a test dose of a short-acting antagonist before the injection. Naltrexone is therefore a choice made at the end of detox, not the beginning, and it requires getting through withdrawal with comfort medications alone.
TIP 63 is careful to say, and we agree, that there is no single right medication. Buprenorphine suits people who want to relieve withdrawal immediately and who can take a daily medication. Naltrexone suits people who want no opioid on board, who work in fields where an agonist medication would be a problem, or who have a strong preference for a monthly injection over a daily dose. What TIP 63 also says, and what the evidence supports strongly, is that any of the three is better than detox followed by no medication at all, which for opioid use disorder is associated with high rates of return to use and of overdose death.
Medication continues into residential.
For years, many residential programs treated detox as the process of getting a person off all opioids, buprenorphine included, and treated a person still on medication as not truly in recovery. That view has done real harm, and it is at odds with every major guideline. At MLJ, medication for opioid use disorder is treatment, and it does not stop at the end of the first week. If you are stabilized on buprenorphine in detox, you remain on it through residential treatment, through PHP and IOP, and into transitional living and alumni care, under the same board-certified psychiatrist who started it. If you choose naltrexone, the first injection happens before you leave residential, so that the transition home is covered. The decision to taper off medication eventually, if it is made at all, is made months later, with your psychiatrist, from a position of stability.
What residential treatment adds to the medication is the rest of the work. The reasons a person came to use opioids, whether chronic pain, trauma, an anxiety or mood disorder, or a professional life that ran on exhaustion, do not resolve because the withdrawal has. The co-occurring conditions that ride alongside opioid use disorder are treated by the same team. Sleep is rebuilt deliberately. Pain is reassessed. And the return to work, to family, and to a social world that may include the people you used with is planned rather than left to chance.
Finally, a word about naloxone. Every client who leaves MLJ after opioid treatment, and every family member who wants it, is offered naloxone and shown how to use it, regardless of which medication they are on. This is not pessimism about your recovery. It is an acknowledgment that opioid use disorder is a chronic condition, that lapses happen, and that with fentanyl in the supply a lapse should never be a death. The medication plan, the naloxone, and the relapse-prevention program are three parts of the same commitment: that you get through this alive and get to do the longer work.
- Medical detox at MLJ — the pillar page: what detox is and what the first week looks like across substances.
- Detox medications — buprenorphine, naltrexone, clonidine, ondansetron and the rest, explained without dosing.
- Polysubstance detox — what changes when opioids are combined with stimulants or benzodiazepines.
- Sleep in early recovery — the symptom that outlasts withdrawal.
- Fentanyl addiction treatment — the program that follows detox.
- Fentanyl: warning signs, risks, and treatment options — for families trying to understand what they are seeing.
This guide is educational and is not a substitute for medical advice. If someone is in immediate danger, call 911.