Animal Reactions May Differ
A prominent argument amongst the pro-animal testing camp is that animals are necessary as they help simulate the effects a certain drug will have when given to humans. Some people feel that this isn't always the case, however. The argument here is that in many cases, animals will not react in the same way as humans, leading to potentially deadly consequences for humans. Supporters of this view point to the case of Thalidomide in the 1960s, for example. Though Thalidomide proved safe when mass tested on animals, it had negative effects on some children, who suffered birth defects after their mothers had taken the drug, as noted by the Coalition To Abolish Animal Testing.
Morals
One argument against animal testing is that animals have the same rights to live a stress and pain-free life, in the same way that many believe humans do. Since animals feel pain and can suffer incredible amounts of stress when locked up and subjected to experimentation, animal testing is morally repugnant to some people. Others believe that the death of an animal through testing should be treated no different from a human death, and that animal testing thus equates in murder.
Religious View
Some people apply their religious views to their take on animal testing. From the point of view of some Christians, for instance, God intended for animals to live a certain kind of life. While humans may be deemed to have dominion over animals, our relationship with animals shouldn't be one of exploitation, but of love, as pointed out by All Creatures. These Christians would, therefore, argue that breeding animals to use in experiments is exploiting them.
Non-Medical Purposes Unjustified
Another argument accepts that animals might be used in testing when the result is to develop a new drug, which could be used to treat disease and thus save human lives. However, some people who agree with this may feel that this should be the extent of animal testing. Experimentation on animals for other purposes, for example to develop a new cleaning product for use in the home, would be wrong, since the cleaning product is a human want, not a need, and thus the potential suffering of animals can't be justified.