In his comments about marijuana legalization to The New Yorker's David Remnick, President Obama draws a line between pot and "harder drugs":
When it comes to harder drugs, the harm done to the user is profound and the social costs are profound. And you do start getting into some difficult line-drawing issues. If marijuana is fully legalized and at some point folks say, Well, we can come up with a negotiated dose of cocaine that we can show is not any more harmful than vodka, are we open to that? If somebody says, We've got a finely calibrated dose of meth, it isn't going to kill you or rot your teeth, are we OK with that?
It depends what you mean by "we." I am OK with that, and so is anyone else who believes people have a fundamental right to control their own bodies, but polling data suggest most Americans are not—possibly because they, like Obama, accept many scary myths about "harder drugs," such as the idea that methamphetamine makes your teeth rot. What would a "finely calibrated dose of meth" look like? Possibly like this. Obama seems unaware that stimulants considered so dangerous that legalizing them is unthinkable are widely prescribed to schoolchildren.
Remnick calls the possibility of broader drug legalizatiion, resulting from consistent application of libertarian principle, a "slippery-slope argument" against repealing pot prohibition. Libertarians, who often warn that the precedent of one government intervention will lead to additional, more ambitious interventions down the road, are used to a different sort of slippery slope, one that leads to tyranny. In Remnick's nightmares (and possibly Obama's as well), it is freedom that lies at the bottom of the slippery slope.