in pictures and a few words
What it is and why it matters
Opioids are strong pain medicines. They are often lined up in a list by “strength”. Let’s work out what this strength means.
- The strongest on the list are sufentanil, fentanyl and buprenorphine
- The numbers come from a large comparison of many studies in people
- These medicines are prescription-only
What “strength” means
Strength is how much medicine is needed. The stronger the medicine, the less of it is needed to give roughly the same relief as morphine.
- Stronger does not mean it eases pain better
- Stronger does not mean safer
- Strength tells you only about the size of the dose
How it was counted
After surgery a person is given a button. When it hurts, they press it, and a machine puts the medicine into a vein.
- Scientists counted how much medicine people gave themselves until the pain eased
- Old reference tables were not used — it was counted afresh
Tested in people. Tested in people: the numbers come from studies in people after surgery.
Strong medicine — a tiny dose
The strongest medicine on the list is needed in hundreds of times smaller amounts by weight than morphine. The dose is counted not in milligrams but in micrograms.
- A counting error costs that many times more
- That is why the strongest opioids belong where there is supervision, not in a home medicine cabinet
Where they are used
In Italy, all packs of the strongest medicine on the list are supplied only to hospitals.
- Fentanyl is most often a patch for pain that lasts a long time
- Buprenorphine: a patch for pain, a form under the tongue to treat opioid dependence
- The strongest of them has a form under the tongue in Europe: prescription-only, given by a health worker, for no more than two days
How the medicine is chosen
A doctor does not choose an opioid by strength. Reviews of studies found no significant difference between opioids — neither in pain relief nor in getting back to everyday life.
- How fast relief is needed and how long it should last
- How your kidneys and liver are and what other medicines you take
- Whether you have taken opioids before
Which is easier to tolerate
Reviews of studies found no difference between opioids in how often unpleasant effects happen — with one exception. With tapentadol they happen less often than with oxycodone.
Tested in people. Tested in people: this is what several studies comparing opioids with each other showed.
Who should see a doctor first
Opioid painkillers are used only as prescribed by a doctor. Benefit and risk depend above all on the dose, not on the strength of the medicine.
- The risk of dependence and overdose exists with all opioids, whatever their strength
- Dependence can start with prescribed treatment — even in someone who has never taken them before
- Switching from one opioid to another or changing the form is a job for the doctor
Important. The danger comes not from which opioid is prescribed, but from how much of it, for how long and under what supervision.
In one sentence
An opioid’s strength is the size of the dose, not the quality of treatment: the stronger the medicine, the less of it is needed and the more supervision matters.
Key points
An opioid's potency shows how many times less of the drug by weight is needed for comparable relief than of morphine; the table sets out what is known about the strongest opioids and where they are used.
| Situation | How it helps | What human studies showed |
|---|---|---|
| Acute pain after surgery, an intravenous pump the patient controls | pain relief on the level of morphine at a dose many times smaller by weight; the dose is in micrograms, so — only under supervision | network meta-analysis of 52 RCTs, 16 opioids: for comparable relief about 423 times less sufentanil was needed by weight than morphine, 58 times less fentanyl, 37 times less buprenorphine (PMID 36523208[1]) |
| Tolerability: tapentadol | adverse events less frequent than with oxycodone | in systematic reviews the frequency of adverse events did not differ between opioids, with one exception — tapentadol shows a lower incidence than oxycodone; four pieces of work, the Cochrane review among them (French guideline 2026, PMID 42371399[2]) |
| Long-lasting pain: fentanyl | almost a hundred registered names in the Italian register, mostly patches — a form for pain that lasts | — |
| Buprenorphine | patches are dispensed as an analgesic, sublingual forms as treatment for opioid dependence | — |
| Acute pain under supervision: sublingual sufentanil (Dzuveo) | authorised across the EU since 25 June 2018; prescription-only, given by a healthcare professional experienced in treating pain, in a monitored setting, for no more than 48 hours | — |
How it helps
The potency figures come from a network meta-analysis of 52 randomised trials covering 16 opioids (PMID 36523208[1]).
| Drug | Times stronger than morphine | 95% CI |
|---|---|---|
| Sufentanil | 423 | 335–533 |
| Fentanyl | 58 | 48.2–68.6 |
| Buprenorphine | 37 | 26.7–50.8 |
| Remifentanil | 13 | 9.4–19.1 |
| Alfentanil | 7 | 4.0–11.0 |
| Hydromorphone | 6 | 5.0–8.4 |
| Morphine | 1 | — |
| Oxycodone | 0.9 | 0.65–1.34 |
| Tramadol | 0.08 | 0.07–0.10 |
All the data come from one device — a pump the patient operates. After surgery a person is given a button: the pain rises, they press, the machine delivers a dose intravenously and then locks for a set time.
The authors did something that makes the work worth reading: they refused to use existing conversion tables. Opioid potency is usually taken from reference tables that accumulated over decades and were copied from source to source. Instead, potency here was computed as the inverted ratio of total consumption — how much of the drug a person gave themselves before the pain settled to a comparable level.
Which gives a simple rule for reading the table: 423 for sufentanil means about 423 times less of it was needed by weight, not that it relieves pain 423 times better. The relief achieved across the compared trials was comparable; that was a condition of inclusion.
Tapentadol stands out for tolerability: in systematic reviews it is the one exception — adverse events are less frequent with it than with oxycodone (see «Key points»).
What potency means in practice
A difference in potency settles a question of dosing, not a question of choice.
When a drug is tens of times stronger than morphine, the work is done in micrograms rather than milligrams, and the cost of an arithmetic slip grows by the same factor. That is why the strongest opioids live where there is supervision, not in a bathroom cabinet.
The choice between drugs is decided by other things: how fast relief is needed, how long it must hold, the state of the kidneys and liver, what else the person is taking, whether they have had opioids before. The potency table answers none of those questions.
According to the systematic reviews cited by the French guideline, no significant differences in efficacy between opioid drugs were found — neither in pain relief nor in the recovery of daily function.
Where these drugs are used
The Italian medicines register shows where these drugs are actually used.
| Drug | Registered names | How it is supplied |
|---|---|---|
| Sufentanil | 4 | every pack hospital-only |
| Fentanyl | 95 | prescription, none over the counter; mostly patches |
| Buprenorphine | 71 | prescription; patches and sublingual forms serve different purposes |
The strongest drug on the list does not exist outside a hospital. Every registered sufentanil pack is classed for hospital supply. A sublingual form for use outside one did exist in Europe: Dzuveo was authorised across the EU on 25 June 2018 — and even that is prescription-only, given by a healthcare professional experienced in treating pain, in a monitored setting, and never for more than 48 hours. A second such product, Zalviso, had its authorisation withdrawn by the European Commission on 5 August 2022 at the request of the marketing authorisation holder, for commercial reasons.
Fentanyl has almost a hundred registered names, mostly patches — a form for pain that lasts, not for a shot on demand. Buprenorphine has split by purpose: patches are dispensed as an analgesic, sublingual forms as treatment for opioid dependence.
Who should see a doctor first
Opioid analgesics are used only as prescribed by a physician. The French national guideline on the appropriate use of opioids, published in August 2026 (PMID 42371399[2]), says verbatim:
«Regardless of their pharmacological potency, the benefit–risk balance of opioid analgesics is primarily dose-dependent, with risks, including the development of opioid use disorder and overdose, shared across all opioids.»
France is among the least affected OECD countries for opioid mortality: an estimated 300–400 fatal overdoses a year, linked primarily to methadone (40%) and heroin (20%), with analgesic opioids far behind at 12%. For comparison, in the United States in 2017 opioids were implicated in two-thirds of 71,568 overdose deaths.
But to stop at that figure is to stop reading too early. The guideline's concern lies elsewhere: a new profile of the person who overdoses has appeared. Not the «typical» drug user, but someone opioid-naïve or an occasional user — someone whose dependence began with prescribed treatment.
That is why the sentence about dose matters more than the table about potency. The danger is not created by which opioid was prescribed, but by how much, for how long, and under what supervision.
Converting between forms and between drugs is a clinical task. The guideline states verbatim: there are no universally accepted opioid conversion tables for acute pain. The recommendation is to work from each drug's own product information, and when rotating from one opioid to another to reduce the calculated dose by 25–50%, because people respond to opioids differently and tolerance does not carry over completely between them.
Bottom line
The numbers in the table are real and were obtained by a good method — possibly the best available, because the authors declined to copy the old reference works and counted again from the trials.
One practical rule follows, and it runs against intuition: the higher a drug sits on the list, the less of it is needed and the less room there is for error. Potency is not might, it is scale. And where the scale is finer, supervision matters more.
Caveats
- This is intravenous only. Every trial used a pump in acute pain. The ratios do not transfer directly to a patch, a tablet or a sublingual form.
- Behaviour, not the molecule. Potency was computed from consumption — the behaviour of a person with a button, not a property of the molecule in isolation. The figures answer a narrow question: how much drug a person with a button needed after surgery. Not «which relieves pain better», not «which is safer», and certainly not «which to choose».
- Short-acting drugs look weaker. The authors write it plainly: short-acting opioids are less potent compared with longer-acting drugs, morphine for example, probably because of the shorter intervals for readministration. Part of the order in this table is about the clock, not about power.
- The results in places contradict the literature. That is not an admission of weakness: reference tables have long been criticised for imprecision, and here they were deliberately not used. The disagreement follows from the method rather than from an error.
- Not a ranking. The guideline considers the classification of opioid analgesics proposed by Lussier and Beaulieu preferable to the WHO three-step ladder with its division into weak and strong opioids, and no clear superiority of «strong» over «weak» opioids has been demonstrated. A strong drug is not a better or a worse drug; it is a drug you need less of by weight.
- Tablets. For oral use the counting is done differently — with a morphine-equivalents chart: 7.5 mg of oral hydromorphone is treated as equal to 20 mg of oral oxycodone, because both are counted as equal to 30 mg of oral morphine. The two drugs were never compared with each other — both were converted through morphine, and the value comes not from a trial but from a teaching chart for emergency physicians (PMID 37465775[3]), which says in as many words that it is extrapolated from the chart rather than measured head to head. In the guideline authors' view, such a chart is not fit to serve as an exact measure — it is a starting point, not an answer.
- Tapentadol. The guideline describes the evidence on its efficacy, in its own word, as heterogeneous: some meta-analyses report superiority over oxycodone, others non-inferiority, and one comparative analysis found it less effective than tramadol; attaching a potency figure to it would invent a precision the sources do not contain. In trials running twelve weeks, treatment was discontinued for lack of efficacy in 6.2% of patients on tapentadol against 3.4% on oxycodone. The authors call both proportions low, and 6.2 against 3.4 cannot be read as «twice as bad»: the difference rests on small numbers. But the direction is reversed: better tolerated, and more often abandoned for not working well enough.
This article is for information. Opioid analgesics are used only as prescribed by a physician; it contains no guidance on doses, regimens or conversion from one drug to another, and none should be inferred from it.
References
Key facts
- The potency figures come from a network meta-analysis of 52 randomised trials covering 16 opioids: relative to morphine, sufentanil is 423, fentanyl 58, buprenorphine 37.
- Potency means how many times less of a drug by weight was needed for comparable relief: it was computed afresh from consumption on an intravenous pump the patient controls, without existing conversion tables.
- Tapentadol is the one exception in systematic reviews: adverse events are less frequent with it than with oxycodone; four pieces of work support this, the Cochrane review among them.
- The French national guideline of 2026: the benefit–risk balance depends primarily on dose, and the risks of dependence and overdose are shared across the whole class — regardless of a drug's potency.
- The strongest opioids are used under supervision: in the Italian register every sufentanil pack is hospital-only, fentanyl comes mostly as patches for long-lasting pain, and buprenorphine patches are dispensed as an analgesic while sublingual forms serve as treatment for opioid dependence.
