Most preclinical studies must adhere to GLPs in ICH Guidelines to be acceptable for submission to regulatory agencies such as the Food & Drug Administration in the United States.
For example, canines may not be good models for solid oral dosage forms because the characteristic carnivore intestine is underdeveloped compared to the omnivore's, and gastric emptying rates are increased.
Also, rodents can not act as models for antibiotic drugs because the resulting alteration to their intestinal flora causes significant adverse effects.
Most studies are performed in larger species such as dogs, pigs and sheep which allow for testing in a similar sized model as that of a human.
However, most research will still involve animal based testing for the need of similarity in anatomy and physiology that is required for diverse product development.