Rats and mice, for example, tend spontaneously to develop a high incidence of tumors. This renders them unsuitable for detecting tumors. Still, no an… - Deborah Mayo

Interspecies differences may lead to concluding that substances that are innocuous or beneficial in humans are harmful, and that substances that have insidious effects on humans are harmless. For example, penicillin is extremely poisonous to guinea pigs. Had penicillin been subjected to the routine animal tests, as new drugs presently are, it would never have been tried on humans.

Those experiments that cannot be justified on scientific grounds can be no more justified than the frivolous killing and torturing of animals. In fact they are even less capable of justification, since such experiments block more fruitful uses of scientific resources.

One type of medical research involves ascertaining whether certain pathological conditions in humans can be alleviated or cured by certain drugs. Animals are used as "models" upon which to test these treatments. To do this it is necessary for the animal subject to have the condition in question, and in order to bring this about healthy animals are made sick. … It turns out, however, that the conditions artificially induced have little in common with the naturally occurring diseases in animals (when these exist) and much less in common with the diseases in man.