If animal testing is supposed to make sure that drugs are safe, why do some new drugs have serious side effects?

Gloved hands holding vaccine

Image courtesy of National Cancer Institute.

Although animal tests help screen out drugs with dangerous side effects, the results of tests with a small number of animals cannot predict what might happen when a drug goes into widespread use. U.S. law requires new drugs to go through pre-market testing involving two different species of animals. Very often a drug that works well in isolated cells is harmful to animals. If the risks outweigh the benefits, the drug will be abandoned. Only if the benefits are greater than the risks will the drug proceed to human clinical trials.

In clinical trials, a small number of people take various doses of a new drug and their health is monitored to determine whether the drug is safe to use and effective in treating disease. If the compound passes this test, it can be put on the market. Drugs for common conditions may be used by millions of people, including some who are very sick, some who are taking medicines for other health problems, and some who may not take the drug the way they should. Animal tests reduce the risk of dangerous side effects, but they cannot guarantee that new drugs will be safe for everyone who might use them.

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