The chosen methods have to fit the purpose and provide sufficiently reliable data for decision making. Today, most decisions are made based on a combination of methods and tools.
Although there are examples where alternatives have successfully replaced the need to use animals, such as for skin irritation, where alternatives have proved to be robust and reliable replacements for whole animal models, unfortunately, scientific progress has been less successful in other areas. The complexities of interactions and responses to substances between different systems of a whole animal have proved difficult either to fully understand and/or to successfully replicate in alternative strategies. In such circumstances it is likely that a total animal replacement would require a battery of alternative tests combined with information through other sources before animal use could be totally abandoned.
For example to study the effects of a substance on a full body system including effects on reproduction or embryo/foetal development, a combination of questions needs to be asked which a simple cell culture will not be able to cope with: How does the substance enter the body? Which organs does it go to? Will it accumulate there or will it be excreted? How long will it stay in the target organs? Does it pass through the placenta? How will it affect a baby in the womb? Which organs of the baby will be affected by it? What will the effects be? Are the effects reversible? Will the effects be different at different stages of the baby's development? These are critical questions for example when deciding whether a medicine can be safely prescribed for a pregnant woman.
When the precise mechanisms of the toxicity of substances are unknown, the use of animals allows the impact on full body functions including reproductive systems to be observed. Replacing animal studies in these more complex areas is a challenge and requires significant further work to understand the underlying mechanisms in order to put the pieces of the puzzle together by using a variety of different tools. The validation of such systems and/or its parts will need to be carefully considered to ensure that the results are reliable.