You were responsible for the welfare of others prior to the enactment of ACA; I presume that you paid taxes for law enforcement, emergency services, infrastructure projects, education, medicare, medicaid, social security, etc., etc. The ACA has not fundamentally changed anything, and the enactment of the ACA does nothing for your argument in a way that restricts one from simply turning the argument around on you by referencing any number of statutes and legislative decrees pronounced by both federal and state governments over the past two hundred some odd years.
Since my taxes serve to support law enforcement and prosecutions, and maintain penitentiaries, it is in my interest, as a taxpayer with vested financial interest and ownership in these projects, that others not be arrested, prosecuted, and detained for harmlessly breaking arbitrary statutes.
And this is where you enter a gray area. "Harmlessly" and "arbitrary statutes" mean different things to different people.
If one wants to break it down in terms of costs and benefits, as opposed to simply respecting an individual's rights to do as he so pleases with his own body, then one has to weigh the costs of enforcement, prosecution, and detention against the costs associated with the negative health effects of using certain drugs.
It is not so evident that the costs of the healthcare would outweigh the costs of enforcement, prosecution, and detention (enforcing drug laws and operating prisons is are expensive endeavors).