This is a difficult question. If humans want to use medicine, we need to check it’s safe before lots of people start using it. If we don’t know whether a medicine will kill people, it is usually tested on an animal first.
People in charge, governments, have decided that an animal life is worth less than a human life. It’s OK to disagree with that if you want to, but if you want a new medicine, it’s a good idea to know whether it can have bad effects on you first.
Actually it is largely down to legislation. Pharmaceutical companies would far rather not use animals and for many areas there are simple tests that can be carried out on petri dishes and on model systems. Eventually however, a potential medicine has to be carried out on a living animal and it is ethically unacceptable to go directly from the laboratory to human beings (although at the outset of the AIDS epidemic, AIDS victims in the US sought a legal ruling to have the right to be experimented on at a very early stage as they knew this was their only – faint – hope). So some animal experimentation is unavoidable I’m afraid. The big problem lies with the government’s insistence on the use of the LD50 test (the quantity of drug that will kill 50% of a quite large number of animals). The problems is, the safer the drug the more that has to be given to the animals. This is a quite stupid regulation but succesive govrnments have refused to modify it.