People who support animal testing argue that almost every medical discovery in the 20th century used animals in some way . They say that even complex computers can not model connections between molecules , cells , tissues , organs , organisms , and the environment . Supporters of the use of animals in experiments , such as the British Royal Society , argue that virtually every medical achievement in the 20th century relied on the use of animals in some way , with the Institute for Laboratory Animal Research of the U.S. National Academy of Sciences arguing that even sophisticated computers are unable to model interactions between molecules , cells , tissues , organs , organisms , and the environment , making animal research necessary in many areas .