During the Reagan administration the Anti Drug Abuse Act of 1986 created mandatory minimum sentences which targeted crack cocaine. Five grams of crack cocaine was a five year sentence and five hundred grams of powdered cocaine drew the same sentence. Crack became very popular with black drug users while powder remained the form of choice among most whites. Simply possession of crack would draw a harsh prison term. While possession of a half kilo of powder would to most seem possession with intent to sell it was treated the same. Very few "recreational users" of powder would ever be holding a half kilo of the substance and thus were spared the mandatory minimum. The legislation was not based on research but on a belief that crack was more addictive than powder and that it's users would be more likely be involved in gun violence. You might want to give a look at Unequal under Law, Race and the War on Drugs by Doris Provine.