I'm not sure what you're even suggesting here. First of all, according to the canadian government[1], euthanasia is not currently permitted in cases only involving mental illness. It's been delayed twice already, currently until 2027, and may be delayed further or scrapped entirely in the future for all we know.

Second, what would they be testing? Drugs for their condition? That wouldn't apply to the type of situation necessary for euthanasia under Canadian law[2]:

>To be considered as having a grievous and irremediable medical condition, you must meet all of the following criteria. You must:

1. have a serious illness, disease or disability

2. be in an advanced state of decline that cannot be reversed

3. experience unbearable physical or mental suffering from your illness, disease, disability or state of decline that cannot be relieved under conditions that you consider acceptable

If there was an experimental treatment that might actually help them, these criteria would not apply.

If you're suggesting safety testing of drugs for other illnesses, that's extremely unethical and I seriously doubt the medical community would be willing to do it, even with consent. And I seriously doubt a number of people large enough to give meaningful results would consent in the first place. It's also of dubious usefulness because any treatment intended for disease X might have its safety profile and risk/benefit analysis altered by the presence or non-presence of X.

1: https://www.canada.ca/en/health-canada/services/health-servi...

2: https://www.canada.ca/en/health-canada/services/health-servi...