Not impacting other people isn't the sole criteria. It's also not the correct premise: not initiating violence and or traditional crime upon other people is the criteria, not a vague 'do not negatively impact other people in any manner.' Libertarian-leaning thought in regards to social liberty does not argue that you should/can have no negative impact whatsoever. It argues you are to not initiate violence upon others.

Legalization and decriminalization are also the sole rational alternatives, broadly, to prohibition. We've tried prohibition every possible way across two generations, and have spent truly vast sums of money attempting it all. It was an extreme and total failure without exception.

Legalization & decriminalization have a minimum of three very strong arguments in their favor. Each of the arguments alone decimates the prohibition argument. These three arguments encompass nearly every spetrum politically, except for most of the very far right traditional wing of old conservatism.

Argument one: it's your body, as an adult liberty means you largely have a right to do with it as you see fit. The government should err to the side of staying out of my life. That's the more libertarian, government leave me alone, argument. Even conservatives should universally go for this one, however they betray the principles they pretend to believe in here.

Argument two: the obvious dollars & cents argument. It's radically more cost effective to treat people with addiction or mental health problems. It's not cost effective to put them in prison at a cost of $40,000 per year (a million dollars over ~25 years). It's also not cost effective to maintain a police state system that costs hundreds of billions of dollars per year. It's like maintaining a second military. That money should be going into healthcare, infrastructure, education, job training. This is another argument on which a lot of conservatives betray their claimed principles: what's more anti small government than a giant bureaucratic police state system? It also destroys the economic value of the person put into prison, making them a non-contributing person and dependent on big government: another thing that should remove conservatives from supporting the prohibition approach.

Argument three: compassion. It's illiberal and very cruel to put a non-violent offender into prison for 15 or 30 years out of their life (drug addicts are often repeatedly arrested), for drug use or drug addiction problems. You know your premise about the opioid lady (or anyone like that in general)? Putting her in prison repeatedly, disappears her from the child's life as well, at a very high cost to society and the child. With the treatment approach, you at least have an opportunity that the parent can be present. It's worth noting here that prohibition does not stop overdoses, it amplifies the lethality of the situation by pushing overdoses into the darkness.