all drugs that are licenced for use in the UK have to be tested on animals.
Without the safety information provided by this phase of testing, they would not be allowed to go on to the next step, which is testing the drug on healthy volunteers (phase 1)
If all goes well with that it is tested (dose ranging) on volunteers how have the disease it will be used to treat.
If all goes well with that it will go into Phase 3 trials, where lots more people take the drug as part of a (normally) double blind clinical trial. The drug will have been tested on a few thousand people at this stage. PG women and children, and often the elderly don't do this bit. The drug then goes to Phase 4, when it is used by the general public (if prescribed naturally) but with added safety precautions, if you've ever had a drug with a black triangle on the box, this is a phase 4 drug.
the company then puts together a huge safety profile and it goes off to the government.
It takes many many years of testing to get the drug onto the ,market and costs around £1000 million