What? The same codeine you can buy without prescription in Tesco?
As for FENT (an assay of Pethidine if anyones interested), which is 50mcg/ml, it has a rapid onset with a short half life. This makes it ideal for chronic and/or paliative pain and analgesia in association with anaesthesia. I give Midazolam followed by FENT, but in a very controlled way and I'm always mindful of knocking their breathing off.
Ordinary FENT has approx 10x the potency of Morphine. However, there are other versions of FENT such as Alfentanyl, Remifentanyl and Carfentanyl (there are others) that are significantly more potent that normal FENT. These substances will stop a person breathing in relatively small volumes (1 to 2 mls) in less than a minute.