Limitations of Animal Tests

Humane Society International

Guinea pig

It doesn’t matter whether you live in a city, suburb, or on a farm, and it doesn’t matter whether you work in an office, a factory, or at home—or whether you don’t work at all. You’re going to come into contact with lots of synthetic chemicals every day, starting with your toothpaste (even your “natural” toothpaste) and shampoo, and going on from there. Synthetic, or human-made, chemicals, along with all of the naturally occurring chemicals, are just a fact of modern life. To make sure those chemicals are used safely, toxicity testing is a fact of modern life, too.

Unfortunately, there are serious problems with the current system of toxicity testing. Some of those problems may not come as a surprise, such as the fact that toxicity testing is extremely expensive, or that it raises tough moral and ethical issues because it harms or kills many animals. But some of the difficulties are less obvious. Like this one: the current approach to toxicity testing just doesn’t work as well as scientists would like.

Animal tests often miss the most important signs of toxicity in humans

When scientists study a chemical, they’re often trying to figure out if it’s safe for a person to be exposed to a very small amount of it for years at a time. After all, you don’t eat a tube of toothpaste every morning and night; you use just a dab, and when you’re done you spit it out. Similarly, you don’t eat the chemicals used to make plastic food containers, but you care about the safety of those chemicals because small amounts may get into your leftovers. In other words, scientists are frequently trying to answer questions about the safety of long-term exposure to low levels of a substance—but it’s impossible to study these long-term effects in animals, since most of them don’t live that long, and toxic effects are usually rare. To try to stake the deck to see these toxic effects, scientists expose animals to much higher doses of chemicals than humans would ever experience – the top dose in experiments generally must show some signs of overdose. Plus, the scientists would like the information much more quickly than an animal’s natural lifetime!

In fact, researchers might use concentrations of the chemical that are thousands of times higher than someone would experience in a typical real-world exposure. The problem is, this approach doesn’t make problems show up thousands of times faster. It makes different problems show up. It’s a little like tossing a rat into a vat of gasoline and then concluding that gasoline causes drowning. The rat dies of other causes before tests can show the neurological damage that can be caused by long-term, low-dose exposure to gasoline. All we can possibly learn from high-dose experiments is what might happen in overdose situations.

Animals are not simply small humans

Another problem with animal testing is that it assumes humans are just giant rats, mice, rabbits, or other experimental animals. Sure, there are some key similarities in basic biology, cells, and organ systems, but there are also differences, and those differences can make a big… well… difference!

Four main factors help determine how chemical exposure will affect an animal: how the chemical is absorbed, distributed throughout the body, metabolized, and eliminated. These ADME processes can vary greatly from species to species – and this can lead to critical differences in the effects of chemical exposure between species. Researchers try to use animals that are a close match to humans in ways that are likely to matter for the chemical being tested. If they’re concerned about potential effects on the heart, for instance, they might choose a dog or pig – because the circulatory systems in these animals are more similar to humans than other animals are. If they’re concerned about the nervous system, they might use cats or monkeys. But even with a relatively good match, differences between species can make it difficult to translate animal results into human risk, especially since small differences in biology can have large effects on ADME processes.

For instance, rats, mice and rabbits have skin that absorbs chemicals quickly—much faster than the skin of humans does. So tests using these animals can overestimate the danger of chemicals that are absorbed through the skin. Even something like how much mucus an animal produces to line its stomach can affect how much and how quickly a chemical is absorbed into the bloodstream.

In practice, researchers’ efforts to find the right animal stand-in frequently fail. Just look at the evidence from the world of medicine – the only place where there is abundant human information for comparison. According to the US Food and Drug Administration, more than 90 percent of promising new compounds fail when they’re tested in humans, either because they don’t work or because they cause too many side effects—but each of those failures had looked good in numerous animal tests, usually in several different species. And everyone has heard of medicines that crash and burn after they make it into drugstores and kitchen cabinets. The arthritis medication Vioxx, the weight-loss drug fenfluramine (the “Fen” in “Fen-Phen”), and the allergy medicine Seldane were all pulled from the market after numerous reports of cardiovascular side effects, such as heart attack, stroke, arrhythmia, and heart disease. Animal tests had given these drugs an all-clear.

Animal tests are time-consuming and expensive, limiting the number of chemicals that can be tested

Animal tests cost a lot of time and money. For example, it takes a about a decade and $3,000,000 to complete all of the animal studies required to register one single pesticide with the US Environmental Protection Agency. And the tests for that single pesticide ingredient will kill up to 10,000 animals – mice, rats, rabbits, guinea pigs, and even dogs. Now consider that there are tens of thousands of chemicals lined up for safety testing in countries all over the world – and new laws in some countries that will require more thorough testing of both old and new chemicals. For example, the Registration, Evaluation, Authorisation and Restriction of Chemicals (REACH) regulations in Europe will require animal testing on all new and existing chemicals produced or imported into the EU in large volumes. Evaluation of each chemical could take millions of dollars, many years to complete experiments, and thousands of animal lives. When you do the math, animal tests just don’t make “cents!”

The high price of animal testing also means companies lose a lot of money and time when the tests don’t accurately predict how humans will respond. As we mentioned above, that happens way more than you think: fewer than 10% of potential new drugs make it through human trials. According to an article in Forbes magazine, drug companies spend an average of four billion dollars to develop a new drug. When the drug fails late in the game, that’s billions of dollars lost.

Objections to animal testing

Even as many industries continue to rely on rickety animal tests, others are facing new laws that prohibit testing certain kinds of products on animals. In the European Union, India, Israel, Sao Paulo, Brazil, South Korea, New Zealand, and Turkey have adopted full or partial animal testing and/or sales restrictions on cosmetics. The United Kingdom went a step further and outlawed testing household chemicals (e.g., cleaning and laundry products, air fresheners) on animals. And more countries are likely to adopt these bans, too, as more and more people object to chemical testing on animals.

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Petra Wegner/Alamy

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