No, it's not libertarian ideology, it is economics, which is the study of human choices. Libertarians generally don't recognise the existence of externalities.
The point is that if fentanyl was free for everybody, almost no one would use it. Even most drug users don't use it deliberatly... and if given the choice between other safer opiates and fentanyl most would choose the safer opiates.
Therefore, fentanyl is the result of prohibition. If you keep going you will end up with carfentanyl instead.
Drug use isn't driven by supply, it is driven by demand. You cannot change the preferences of humanity through ideology like prohibition. This will get worse not better because of prohbition.
The Iron Law of Prohibition says that fentanyl is entirely the result of prohibition. Prohibition caused this crisis, it cannot be the solution to it.
Try reading it again, it's not a libertarian philosophy but a deep economic analysis of the incentives behind the epidemic you are advocating for.