The FDA has a number of ways of getting around this for drugs that have limited effect. This is for drugs to be used in the general population, drugs for major illnesses.

Orphan drugs are treated differently: http://en.wikipedia.org/wiki/Orphan_drug

The FDA also maintains a list of pre-approved substances that can be used without testing. I think this is it: http://www.fda.gov/Drugs/InformationOnDrugs/ucm129662.htm