No, I'm thinking of diamorphine. I'm not questioning that fentanyl is also used, but so is diamorphine, and the point was not that it is used a lot, but that it
is used enough that we have reliable data on the cost of various doses of it.
Here are some pages covering various uses by the NHS
Most relevant to the point I made, here is a pricing page for diamorphine for the NHS[1].
Use during birth: [2]
"This is an injection of a medicine called pethidine into your thigh or buttock to relieve pain. It can also help you to relax. Sometimes, less commonly, a medicine called diamorphine is used."
Use in NHS Scotland [3]: "Unlicensed intranasal diamorphine has been used in the NHS in Scotland for the treatment of severe pain in children in the emergency setting. The availability of diamorphine hydrochloride nasal spray (Ayendi®) provides a licensed preparation."
Article on recent-ish supply problems[4] due to a problem at one of the manufacturers:
"NHS trusts forced to use ‘expensive’ pre-filled diamorphine syringes to manage ongoing supply issues
Exclusive: Birmingham Women’s and Children’s NHS Foundation Trust has had to source pre-filled diamorphine syringes from other NHS trusts to meet demand."
NICE advice on diamorphine use [5] lists indications and doses for acute pain, chronic pain not currently treated with a strong opioid analgesic, acute pulmonary oedema, myocardial infarction.
NHS patient information (word doc) [6] on the shortage: "Diamorphine is licensed to treat severe pain associated with surgery, heart attack or a terminal illness. It is also used for the relief of shortness of breath in severe heart conditions. In addition, a small number of people in the UK may be receiving diamorphine to manage heroin addiction."
So while I have no idea about the actual volume of use, as you can tell the NHS has suppliers, and NICE has a price list.
[1] https://bnf.nice.org.uk/medicinal-forms/diamorphine-hydrochl...
[2] https://www.nhs.uk/pregnancy/labour-and-birth/what-happens/p...
[3] https://pharmaceutical-journal.com/article/news/nhs-trusts-f...
[4] https://www.scottishmedicines.org.uk/medicines-advice/diamor...
[5] https://bnf.nice.org.uk/drug/diamorphine-hydrochloride.html#...
[6] https://www.england.nhs.uk/south/wp-content/uploads/sites/6/...