Don't bring it up.
Unless someone asks about his view on the legalization of drugs, I won't bring it up.
Probably the best policy. Just like you don't have to bring up abortion when talking with liberals. It's not necessary to go through the whole laundry list of Dr. Paul's "positions" on "issues", because his campaign, unlike all the others, is
not merely a list of positions hastily stitched together. It's all based on a single principle: the Golden Rule. Peace, Freedom & Prosperity; if they want to get into the details, then you can try to explain how freedom for you has to include freedom for me -- even *gasp* freedom for people we (in our acknowledged superiority) might not like.
As for the "drug issue", the example of Prohibition ought to suffice. If they don't "get it" from that, they're unlikely to be willing to hear the truth at all.
"Drugs" are already legal -- only some are more legal than others. As a former marijuana smoker myself, I always found all the alcohol, nicotine and caffeine addicts' criticisms ridiculously hypocritical. I don't use any of them now, and wouldn't recommend any of them, but the libertarian stance is very clear: the government has no right to interfere in anyone's personal life so long as no harm is being done to others.
On the other hand, if they really want to make drugs illegal, then the only consistent approach would be to make
all drugs illegal -- including
their favorite. Almost everyone in America is addicted to caffeine, which as it is traditionally used is hardly different from the traditional use of cocaine: leaves of the coca plant chewed by South American natives as a pick-me-up. If coffee were illegal, we'd be seeing concentrated caffeine powder sold on streetcorners for shooting up. And all the "best" families would be neck-deep in its illicit trade.