Re: Why do scientists use animals to test medicines and other substances?

Scientists use animals to test medicines and other substances for a variety
of reasons that primarily boil down to whether these substances are
suitable for use. I purposely did not include the words "in humans" because
medicines for pets & livestock are also tested in animals.

Safety is the primary concern, for example whether the
substance can kill you, cause cancer or damage the liver. For vaccines
there is the
additional need to know if the vaccine will protect animals against the
disease.

For some substances there is the ability to do tests that do not involve
animals, but at this time our knowledge of biology is such that it is
impossible to use simple cell culture tests or computer models to predict
exactly what will happen for most substances when they are given to a
person or animal.
Animal testing is currently the best  means of assessing the safety of a
substance. But animal testing is not perfect - a good scientist will make a
careful determination whether the animal they have chosen for testing is a
good one.