Other [denaturing formulas] used bitter-tasting compounds that were less lethal, designed to make the alcohol taste so awful that it became undrinkable.
This approach was also applied to non-industrial alcohol products, leading to another round of mass poisoning:
Which bears an eerie resemblance to the problems we have with methamphetamine today. Much of the damage caused by methamphetamine abuse is the result of adulterants in the drug, which is produced under poorly controlled conditions. Pharmaceutical methamphetamine -- the kind that doctors sometimes prescribe to children -- causes far less damage to its users and even to those who abuse it (not to deny that abusing it is dangerous even without the adulterants).
This approach was also applied to non-industrial alcohol products, leading to another round of mass poisoning:
http://en.wikipedia.org/wiki/Jamaica_ginger