From what I understand, fentanyl can be used in some instances, although it's pharmacology is quite different from morphine. I'm not sure how effective it is in some terminal stomach cancer situations where morphine is often ineffective. If the patient wishes to be conscious and coherent during periods of severe pain management, fentanyl as an analgesic, in many cases would be less effective than heroin.
The friend I mentioned in the thread started by juice_soldier (about his mum) has since been prescribed fentanyl patches. The patches are designed to last 3 days. While the pharmacokinetics are somewhat different for patches compared to other forms of administration, the pharmacodynamics are the same. My friend is fine for the first two days. On the third day he's in agony again, despite the fact that if anyone else uses the 'used' patch it's still very effective. It would seem his tolerance is also increasing, despite only having been on fentanyl for a few weeks. Fentanyl patches (Durogesic) are designed to last ~72 hours, where upon plasma levels fall quite rapidly. When he receives a new patch, due to pharmacokinetics, plasma levels rise more rapidly than from the earlier dose. There are also other aspects of absorption and clearance which makes it harder to control a steady state effect from fentynal.
As fentanyl is a CYP3A4 inhibitor and is a major substate for this isozyme, it's probably not suitable for patients who are also on medications which use this metabolic pathway, such as with some forms of cancer and HIV/AIDS treatments e.g. protease inhibitors. Morphine on the other hand doesn't metabolise via 3A4, as it's primarily metabolised by phase II glucuronidation (and slightly via 2D6). However, in saying that, in a great many cases, fentanyl is used in pain management in terminally ill patients.
Because heroin is more lipid soluble than morphine, it crosses the BBB more rapidly than morphine, making it faster acting. Heroin also has a shorter half life, and it's suggested it might be less emetic than morphine. Due to these variations in effects, smaller doses are required (than morphine) meaning the patient is often more coherent for a similar analgesic action from morphine or even fentanyl. All these drugs can cause severe respiratory depression, so low required dosage and fast clearance is often a distinct advantage and definitely better facilitates communication during periods of severe pain often experienced during the final days of life.
Heroin is still used in Britain and is list by Rang et al as: "Not available in all countries. Considered (irrationally) to be analgesic of last resort"
[Edit; spelling, grammar etc...p_d]
