I don’t how they could allow it in the first place - mental illness should be a mitigating factor in access to euthanasia.
If you’re not mentally well, you can’t properly consent and if you can’t consent it shouldn’t happen. (This is distinct from terminating life support for someone in a coma)
Some people with extreme forms of depression don’t respond to treatment. I dont mean a bit sad or down, I mean debilitating depression that severely affects quality of life and makes living miserable.
If someone asks for assistance in dying there is tests used to prove they know what they are asking for and that this is their emphatic will. If there is no sign of mania or psychosis then there’s no reason to doubt their ability to consent.
See this example.
The counterpoint is that mental illness can be every bit as dibilitating, intractable and painful as physical illness.
There’s a long history of mental health being brushed off and seen as less significant than physical health which has caused a lot of personal and social problems.
The counterpoint to that is that people can recover from mental illness in a way which is not always possible for physical illness, the type of illness which is most relevant to euthanasia legislation.
The counterpoint to that is our societies rarely provide what people need to recover from the most serious, or even moderate, mental health issues.
So there’s some contradictions built into this within our cultures. Personally I come down where you are, that it’s a bad idea to make permanent choices without a sound mind.
Not every mental illness affects your grasp of logic or perspective on reality. Obsessive compulsive disorder can be absolutely debilitating even though the person might completely understand the reality of their situation, know that they don’t actually benefit from and in fact are harmed by, for example, checking the windows and doors every night in the exact same way many times, but still be unable to stop themselves. They aren’t delusional, just prisoner to compulsions they can’t resist. I’m not suggesting that OCD is always severe enough to make someone consider euthanasia or is always resistant to treatment, but it certainly could be.
I could also see someone with substance abuse disorder that they’ve tried and failed to treat for decades making that decision (while sober after detoxing and withdrawal). I would want them to keep trying, because I know how great sobriety and how valuable a life built post addiction can be, but I wouldn’t consider them to be of unsound mind in that scenario.
I’m just listing examples in my family, but I’m sure there are others.