http://www.erowid.org/chemicals/amt/
http://leda.lycaeum.org/Chemicals/AMT.168.shtml
http://www.pooterland.com/index2/looking_glass/merry_pranksters/body_merry_pranksters
... for a clear discussion of two prominent proposals. Examples of existing "digital rights management" and "copy protection" systems: Serial copy management system[?] ...