Just make sure all the dangerous stuff can only be administered in licensed venues by trained professionals.
Make users fill out a 12 page disclaimer before they are served. Make sure that the centers are decorated as tastefully as the average welfare office.
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.