There is also the argument that the reaction of a drug in an animal's body is quite different from the reaction in a human. The main criticism here is that some believe animal testing is unreliable. Following on that criticism is the premise that because animals are in an unnatural environment, they will be under stress. Therefore, they won't react to the drugs in the same way compared to their potential reaction in a natural environment. This argument further weakens the validity of animal experimentation.