Principle bends to pragmatism. It's illegal to kill other humans, except when it isn't (accidents, self-defense, military action, abortion). There's nothing straightforward about the lines we draw between when it's okay versus not okay to kill people. People are entitled to trade their labor freely, except when they aren't (minimum wage, child labor).

Humans are social creatures; they are interdependent cogs in a machine. So as an overarching principle, society is entitled to regulate to interactions between people. To make that bearable, we recognize certain "fundamental rights" that society cannot infringe. Procreation is a deeply-held one, which is why society cannot force women to be on birth control. Taking drugs is not one of those rights.