"Just make sure all the dangerous stuff can only be administered in licensed venues by trained professionals."

Has that stopped dealers from selling Vicodin? Extacy was also legal at one point and it didn't stop anyone from using it as a club drug.

"The hardcore addicts will flock to the centers, which will kill the street trade. But casual users and people looking to experiment won't bother; and they won't be able to find street dealers, as the dealers can't make a profit off the addicts anymore."

There will always be a black market..even if it's legal. This is because many people won't want to have to go to a doctor...and it will still be the most convenient.

I also would be interested to see how a doctor can ethically give you a prescription for a substance that does absolutely nothing to benefit you.