Using animals for medical research is a complicated ethical issue. You could argue that no living being should be subjected to experiments without its consent. There is something morally repugnant about confining an innocent animal and forcing it to endure possible pain and suffering in the name of science.
But can it be justified or at least balanced by the potential for medical advancement? There is no question that humans live longer and suffer less because of research done on animals. Artificial hearts, insulin treatment for diabetes, and breakthrough cancer treatments have all come about at least in part from animal testing.
Above is a slideshow of various animals used in biomedical research, the advances they made possible, and their similarity to humans.