In the conditioned taste aversion test, rats are given two water bottles from which to drink. One is filled with plain water and the other is filled with water to which a small amount of saccharin or sucrose has been added. After a conditioning period, the rats are injected with either a test drug or a saline solution.
The rats are returned to their cages and observed to determine whether they avoid the water bottle filled with sweet-tasting water, which would suggest a learned aversion to the solution, i.e., they associated the sweet-tasting water with a negative reaction (e.g., gastrointestinal malaise) that was actually caused by the injected drug.
Conditioned taste aversion is a useful test for determining whether a drug causes gastrointestinal side effects.