I completely agree to you: a terminally ill person should be given whatever substance they want and even advised to take whatever a drug if it can make their last moments happier. Even if it's addictive or harms health - they don't need to maintain healthy living and society integration any more so why not?

I wouldn't even limit this to patients which are actually going to die soon and have no choice. Any patient eligible for euthanasia should be allowed to take any substance they want. It is absurd to let a person die but forbid them to take a shot of a substance "because it can harm their health" or whatever. I would even suggest them to take psychedelics - this may (or may not - a proper setting can be important) bring peace of mind and even sense of meaning in both dying and living, they pretty much may even change their mind.