Why do they use rats for animal testing?

Answer 1

In the U.S., the number of rats and mice used have been estimated at 15-20 million. Mice share 99% of their genes with humans. Rats and humans respond similarily to many medications, chemicals, etc.

Answer 2

Science labs don't just use rats, they use other animals too. The reason they use animals for testing is because we are animals, too, so the test results can give us information about ourselves. But, on the other hand, rats and other animals are not like us, so they don't have to pay the animals like they'd have to if they tested on people.

Scientists sometimes use rodents (like rats) for experiments - such as the famous ear grown to the back of a mouse experiment - because they are simply easy to find. (But products cannot be tested on any type of animal - like, if you get a product and it says that it was not tested on animals.)