Their first priority is the reduction of human suffering. The prevention of animal suffering or death, although still important, is secondary to that. Allowing animal suffering is justifiable if the certain experiment will have an outcome as an important medical breakthrough. If it is possible in practice they try to keep animal suffering to an absolute minimum by using anesthetics and keeping them in comfortable, airy and healthy conditions with plenty of space to move around. Scientists and veterinarians are always around looking after them. If the suffering cannot be stopped, the law requires that animal must be painlessly killed. In short, idealistic people are trying to experiment on animals without being cruel to them.
Without testing medicines on animals, the only way of testing would be to directly try it on humans. Of course, this testing would put human beings at considerable risk and only few people or even scientists working on medicines would take such a high risk. Millions of lives are saved every year because of animals. Therefore, there are, indeed, some benefits really worth testing medicines on animals first. But is it right to do animal research? Is animal life less worth than human life? On ethical ground, those who oppose animal experimentation say that harming one species in hopes of benefiting another is morally wrong. .
Human benefits can not provide moral justification because of the high number of involved animals and the high level of suffering. It is said that pain is a mental process, something that can be felt, not observed. Due to external indications such as crying, screaming, or moving away from something we can only assume that other human being feels pain. Some say that animals do not feel pain. But if you poke animal, step on it or burn it, it will yelp, cry and move away. Indicators are the same and therefore we know that animals feel the very same pain as humans.