The use of animals in scientific research has been a controversial issue for some time. While supporters argue that most major medical achievements have involved the use of animals, and could not have been made otherwise, detractors of the practice say that it is cruel, unnecessary, scientifically unreliable, and both ethically and morally abhorrent.
The issue of animal testing largely revolves around ethical problems: the justification of using living animals in research to benefit humans, and the problems posed by situations where animals suffer as a result of research.
Another prominent issue is one of compatibility between species: historically, there have been situations where the failings of animal experimentation have been exposed due to differences in the way animals and humans respond to various substances. (Note, however, that the thalidomide story is not such a situation – this was more a failure of scientific design than of the use of animals, as thalidomide was never tested in any pregnant organism before its release for human use.)
Finally, there is the issue of alternatives to animal research, and whether they are viable replacements for any or all types of animal experimentation.