I would disagree. A conservative also must use common sense and isn't above using govenment as a necessary evil in extreme cases. What you are describing is libertarianism or anarchy which in extreme cases are the same thing.
Are the extreme cases regulating what people do to their own body, on their own time, and on their own dime?
Obviously no one should be driving stoned any more than driving drunk, etc; and they are unemployable because they can’t refrain from something - be it booze, cigarettes, weed or coffee- to hell with them, let ‘em starve.
But if someone makes it their business to interfere with a person who wants a drink, or a bong hit, or simply relief from a level of pain *THEY* consider too high, then in my opinion that person is not a conservative, they are an authoritarian who merely wants to use the state to enforce their particular beliefs.
Respectfully
Is there an "extreme case" here?