If heroin overdose is so mortal, would it be a reasonable way to do euthanasia? Would it be a good death? Or it can be in some way painful? It's better for euthanasia than phenobarbital?
According to a 1999 paper
Recent research, mainly from the Netherlands, has shown that high doses of barbiturates are usually effective for physician-assisted suicide [...] Opioids are less reliable drugs for physician-assisted death because of the unpredictable duration of the dying process even after high doses.
There's also a story in the Atlantic about the search for substitutes in the US due to the astonishing price (in the US) of the barbiturates typically used. The story notes...
They discussed the synthetic opioid fentanyl, but were deterred by the drug’s newness and dangerous reputation. So the group decided to use a combination of medications, and eventually settled on high doses of three: morphine, diazepam—also known by its early brand name, Valium—and propranolol, a beta-blocker that slows the heart. They called the mixture DMP. [...]
The first two deaths went smoothly. But the third patient, an 81-year-old with prostate cancer, took 18 hours to die [with DMP]. In Oregon, where aid in dying has been legal for 20 years, the median time from taking the medication until death is 25 minutes.
Patients themselves typically become unconscious in five or 10 minutes, so they are not affected by protracted times [...]. But longer waiting periods can be nerve-racking for families and other caregivers, especially in the exceptional cases where these have persisted for a day or more.
Interesting enough perhaps, barbiturates were also the drug of choice for physicians who killed themselves in one study in England and Wales... but not among anaesthetists.