You may wonder why two different animal species are used for testing at this stage. The reason is that no animal is exactly like a person in every way. A drug may not be toxic to rats but may be toxic to guinea pigs, and, by using two different species, the chances are greater that the toxicity of a drug will be discovered before it is ever given to a person.
This stage of safety testing
usually takes about 4 years. Drug companies test for mutagenicity (ability to cause genetic changes) and carcinogenicity (ability to cause cancer). The drugs are also tested to confirm that they do not cause infertility (inability to have children) or birth defects. This stage of safety testing takes many years, because it may take a long period of time for animals to develop cancer or infertility as a result of a toxic drug.