A key aspect of the animal welfare regulations is that pain and distress be minimized whenever possible. Therefore, it is necessary to design and perform experiments in such a way as to prevent the animals from experiencing problems unless it is necessary to achieve the goals of the study (i.e. category "c" studies).
It is not sufficient to merely address these issues in a protocol. The animals themselves must be monitored, and appropriate actions taken if pain or distress is observed. The problem is that assessing pain and distress in animals is not a simple task.