Often they can both be used, especially in spoken English. However, in this case 'when' is more ambiguous without more context. If the sentence was 'this is not euthanasia, where a doctor gives a patient a lethal injection', it clearly means that if a doctor gives a pateient a lethal injection, then that is euthanasia. With 'when', though, it probably means the same thing BUT it could also mean 'when a doctor gives a patient an injection, it is not euthanasia (completely the opposite). To clarify, we would need to know a little more of what was said before and after.