Organisms are said to be drug-resistant when drugs meant to neutralize them have reduced effect. When an organism is resistant to more than one drug, it is said to be multidrug resistant. The most prominent example of this is antibiotic resistance. Drug resistance is also found in some tumor cells, which makes it more difficult to use chemotherapy to attack tumors made of those cells. When a drug such as an antibiotic is administered, those which have a genetic resistance to the drug will survive and reproduce, and the new population will be drug-resistant.

Immunology | Pharmacologic agents | Oncology