It's a very, very complicated subject.
I have a friend who develops pharmaceutical drugs. He knows I am very anti-testing when it comes to cosmetics / household cleaning products etc... I accept that pharmaceutical testing is a complicated grey area.
In the UK it is highly regulated. Breeding / transporting / conditions under test / euthanasia is all monitored. Partly to ensure welfare standards. Partly that these animals are worth £X amount in terms of the R&D total spend. Also, you can't test on an unhealthy animal or one that don't meet the test parameters. Your test would be invalid / not reliable.
However, lots of research is done out of the U.K. (see Monkey World link), where regulations are not as stringent as in the UK. UK/ multinational companies do use non UK research testing.
Animals can be used for multiple research testing - different drugs for different conditions.
Welfare of the test animals covers the physical but not mental. We're not just using fruit flys or mice. Depending on the length of testing / incubation / longitudinal type studies, these animals can be alive a long time - in isolated conditions. Never out of their cages unless it's for testing. They can and do suffer mentally.
Researchers are learning more and more about animal behaviours / physiology. For instance, all octopi / squids now must be given pain relief before being cut. They have a fully developed central nervous system which of course means they feel pain. Why on earth did we think it was right to cut them without pain relief before??? Also animal physiology and therefore reactions to drugs are different animal to animal (including us). As an example, Leishmanisis is curable in us, but not in dogs. We're both mammals - so what's the difference?
Many drugs that are developed never meet the threshold for animal testing. Many go through and fail at this stage.
More worryingly are the ones that go on to human test stage and then fail and the ones that pass human testing and fail once in use commercially.
My line in the sand is no tested cosmetics / personal care / household cleaning products.
Do I need my whites, whiter? Do I need my hair shinier? Why does bleach need to be re-tested? I also use non- test dog food manufacturers.
But I do not condone violence against researchers / labs. Which is why labs are now (mostly) underground. Building contractors have heavy non talk clauses and Huntingdon Life Sciences have a non identifying marker for the London Stock Exchange.
If you have any investments / pensions etc... chances are your money is being invested into pharma ie research (testing).
www.bbc.co.uk/dorset/content/articles/2009/03/24/monkey_world_capucin_one_year_on_feature.shtml
www.lushprize.org/wp-content/uploads/Lush-Prize-animal-testing-failures-2015.pdf