Per unit fentanyl is far more lethal than other typical opiates
Fentanyl is in the same class of medical compounds as heroin (medically known as diamorphine) and is also used in medially supervised pain relief. Both are opioids and both are dangerous because of the consequences of taking too much. The medical guides (this is from the British National Formulary or BNF) give this general warning about opioids:
Opioids (narcotic analgesics) cause coma, respiratory depression, and pinpoint pupils. For details on the management of poisoning, see Opioids, under Emergency treatment of poisoning and consider the specific antidote, naloxone hydrochloride.
Stoping breathing while in a coma is not usually regarded as an acceptable side effect of a pain-relieving drug so recommended doses are set to minimise that risk. The problem being that the effective window where the patient gets relief from severe pain but doesn't stop breathing is narrow (one reason why using illegal sources where the concentration is uncertain is very risky).
But we can gauge something about the relative "strength" of fentanyl from the medically recommended starting dosages of it and heroin when given for acute pain (these figs are from the BNF):
- heroin: 5mg every 4hr
- fentanyl: 12 𝝻g every hour
So the dose rate for fentanyl is about 1% that of heroin by weight (crudely suggesting that fentanyl is at least 100 time more potent than heroin.
The warning sections in formularies spend a lot of time on getting the dose titration correct because of risk of overdose and fentanyl seems to be more problematic than heroin (which is not exactly easy to start with).
So even under medical supervision fentanyl is dangerous because of the risk of overdose. The amount that will actually be fatal, though, depends on the patient and will be higher in a patient used to the drug than in a fresh user. On any measure though, fentanyl is far more potent than heroin which makes it especially dangerous in unsupervised hands or when sourced illegally as the dosage will vary unpredictably and dangerously.
Note. For those who have trouble accessing the BNF link (there are some restrictions on access), you can get the content as an iPhone app and in print. For other ways to access the information see the wikipedia page.