In the UK, all new medicines are tested on animals before they are tried in humans.
Medicines to treat illnesses such as rheumatoid arthritis, HIV and cancers have all been tested in animals. Medicines for less serious diseases such as the common cold and headaches have also been tested on animals.
If the medicines pass animal tests then they usually help thousands and sometimes millions of people.
But in order to help these people, animals have to suffer and sometimes have to be killed. Scientists must minimise this suffering and follow strict rules to ensure the animals are cared for.
The key question to ask is whether it is right for animals to suffer in order to help human beings and save human lives.
Did you know?
Almost 3 million animals were used in scientific research in the UK in 2005.