It’s common to perform cytotoxicity tests in lab environments. If something is cytotoxic, then it is highly damaging to at least some cells. The cytotoxicity of a given compound will vary. In some cases, it will just be harmful to the cells. In other cases, it will completely destroy them.
Cytotoxicity is complicated, and people should try to remember that even some forms of therapy will be toxic to the cells. Radiation therapy falls into that category, and so does chemotherapy. In this case, those therapies are used to kill off cancer cells, but they will kill healthy cells as well as dangerous cancer cells. This is one of the reasons why these therapies are generally only used to treat the most dangerous forms of cancer.
A cytotoxicity test will be performed for many different reasons. Generally speaking, these tests will be performed on cells that were produced in lab cultures. Researchers will administer these tests in order to get a sense of how a given set of chemical compounds will affect the lab culture’s cells. Some of these compounds could actually make the cells healthier, but situations like that are rare.
Cell division is a delicate process. Making even small changes to the chemical environment can be disastrous. In many cases, cytotoxicity tests will indeed demonstrate that a given compound is dangerous to cells.
These tests are often used to get a sense of whether or not the chemical compounds in question will harm organisms. Often times, researchers will not have a lot of options when it comes to running these tests. They absolutely have to test the chemicals first, and they cannot test them on the organisms themselves.
Some people will criticize the cytotoxicity tests performed on lab cell cultures, pointing out the chemicals themselves will often have different or more complex effects on the bodies of organisms. However, these tests still provide the sorts of valuable evidence that researchers will often need in many contexts.