I don't think it's 'blurry' whether one potentially harms oneself or whether one harms another person.
It can 'blurry' whether what one does to themselves is harmful or not, which may be an interesting debate, but that isn't the focus of that particular piece.
It is very blurry. Lots and lots and lots of examples. But let's stick to the topic.
If I distribute something to you that will harm you, then have I committed a crime or have I just contributed to your vice?
Please name the mythological products that cause zero harm to their users. I'm collecting such a list and so far it is empty.