Naloxone

Is naloxone safe?

Yes. Naloxone is one of the safest medicines a bystander can give, and the danger in an overdose is almost always the opioid, not the antidote. It works by pushing opioids such as heroin, fentanyl or morphine off the receptors that slow breathing. In someone who has taken no opioids, the US label says it has essentially no effect. Giving it when you are not sure what happened is therefore unlikely to do harm.

There are two catches. In someone who uses opioids every day, it can cause sudden, unpleasant withdrawal. And its effect often ends before the opioid's: its half-life is about two hours, and how long it works depends on the dose and route. So a person who wakes up can still stop breathing later, and emergency help is still needed.

We score naloxone 88, one of the highest scores among the medicines we rate. That number describes the drug's own risks. It does not mean an overdose is safe once naloxone has been given: call emergency services every time.

What happens when someone wakes up

Naloxone acts within minutes, and the reversal can be abrupt. A person who is physically dependent on opioids may go straight into withdrawal. The signs include shaking, sweating, vomiting, diarrhoea, a racing heart, anxiety and sometimes anger or aggression. The over-the-counterOver-the-counterA medicine you can buy without a prescription, in a pharmacy or shop, with directions on the pack for safe use without a doctor.More in the glossary Narcan label tells bystanders to expect this. The injection label says the symptoms usually settle within about two hours.

How often does it happen? A 2016 review of take-home naloxone programmes counted 2,336 uses by lay people. It found withdrawal symptoms in 2.8%, vomiting in 2.2%, agitation in 2.1% and four seizures. In hospital use, rare cases of dangerous heart rhythms, fluid on the lungs and cardiac arrest have been reported, mostly after surgery in people with heart disease.

The more naloxone given, the harder the withdrawal tends to be. In a study of 1,831 overdoses treated by Pittsburgh ambulance crews, lung complications (mostly inhaled vomit) were more common when the total dose went above 4.4 mg. The overdose itself causes many of these problems, so the link is hard to untangle. It is one reason the label gives one spray at a time, not several at once.

Newborns are the exception. The label warns that withdrawal can be life-threatening in a baby whose mother used opioids, so hospital staff use doses they can adjust.

Why one dose may not be enough

The half-life of naloxone after the 4 mg nasal spray is about two hours, and the Narcan label warns that most opioids act for longer. The labels for methadone, slow-release OxyContin tablets and fentanyl patches all say that naloxone's reversal is expected to wear off before the opioid does. UK guidance adds that potent opioids such as fentanyl and nitazenes can need more doses. The Narcan instructions follow from this:

1. Check that the person does not respond to shouting and a gentle shake.
2. Give one spray into one nostril.
3. Call 911 in the US or 999 in the UK.
4. Wait 2 to 3 minutes. If there is no response, give another spray with a new device in the other nostril, and keep going until the person wakes.
5. Stay until help arrives, and give another dose if the person becomes very sleepy again.

Put the person on their side while you wait. Buprenorphine binds so tightly that the label says larger or repeated doses may be needed and reversal may be incomplete. The injection label adds that naloxone does not reverse breathing problems caused by non-opioid drugs, so it will not undo alcohol or sedatives taken with an opioid.

Where to get it

In the US, the FDA approved Narcan 4 mg nasal spray for sale without a prescription on 29 March 2023. RiVive 3 mg followed that July. The CDC says it is sold in pharmacies, convenience stores, grocery stores and petrol stations, and that community naloxone programmes and most syringe services programmes give it out.

In the UK, naloxone is still a prescription-only medicinePrescription-only medicineA medicine that can be supplied only with a prescription from a doctor or another authorised prescriber.More in the glossary. Since 2015, drug treatment services have been allowed to give it out without a prescription, and nasal sprays such as Nyxoid were added in 2019. Regulations from December 2024 extended this to police forces, prisons, probation and youth justice services, armed forces medical services, nurses, midwives, pharmacists, pharmacy technicians and paramedics. Anyone may use naloxone to save a life in an emergency. A consultation on supply by homelessness services and public emergency naloxone boxes, like defibrillator cabinets, closed on 9 March 2026.

Naloxone or nalmefene?

Nalmefene (Opvee) is a longer-acting antagonist. The FDA approved the nasal spray in May 2023, on prescription only and for people aged 12 or over. Its half-life is 11.4 hours on the label, against about two hours for naloxone. Even so, the Opvee label still warns that breathing problems can return. It also warns about attempts to overcome the block by taking more opioids. Naloxone remains the antidote that most kits contain and that bystanders can buy freely.

How we scored it

Our medication scores measure how safe a drug is when used as the label directs, and naloxone does very little harm on its own. The score covers the antidote, not the overdose, and emergency help is still needed. Dependence (98), track record (95) and overdose marginOverdose marginHow far a normal dose is from a harmful one. A wide margin means an accidental extra dose is unlikely to cause serious harm.More in the glossary (92) are near the top of the scale. Serious side effects (80) reflect withdrawal and rare heart and lung events, and interactions (85) reflect that it blocks opioid pain relief and maintenance treatment. We do not mark it down for wearing off before the opioid, because that is a limit of the drug's action rather than a harm it causes. The page covers it at length for that reason.

If you live with or care for someone who takes opioids, including prescribed ones such as fentanyl patches or methadone, keep naloxone where others can find it and show them how to use it.