Many people consider animals to be creatures holding almost the same dignity as human beings, and worry about their well-being. Unsurprisingly, some are bothered by scientific experimentation on animals to the point of wanting it to be abolished. Meanwhile, others stress that employing animals in research can lead to important knowledge breakthroughs. Despite believing animal experimentation to be a very unfortunate procedure, I sustain that it is justifiable and the best alternative available, although by no means I consider it a good enough way of doing things.
To justify it, I would approach the question in regards to the importance of lives, especially human ones. When correctly performed to attain noble goals such as curing grave diseases or relieving the suffering of terminal patients, animal experimentation can lead to major advances and save millions of lives, a sacrifice that outweights the loss of the laboratory subjects.
Rats, for instance, have been experimented on for decades now, having helped to discover the key functioning mechanisms of numerous health conditions. Therefore, I would argue that researchers are right to pursue such advancements even at the cost of animal sacrifice. However, I strongly disapprove of research efforts which only seek to deliver commercial results instead of ethically desirable solutions. It is not acceptable to sacrifice lives for the sake of superfluous cosmetic solutions, for example.
Even so, one must never forget that animal experimentation is a somewhat primitive strategy that mankind should seek to surpass as soon as possible. While finding suitable substitutes to complex living organisms is by no means an easy task, one could expect that modern life sciences will become increasingly capable of employing computational models to simulate valid experiments, thus progressively diminishing the need to sacrifice actual living beings.