the ability of a pathogen to withstand a number of antimicrobial drugs — such as those that might have fought that pathogen before.

resistance of tumor cells to several unrelated drugs after exposure to a single chemotherapy drug.

A characteristic of cells that makes them resistant to the effects of several different classes of drugs. There are several forms of drug resistance.They each are determined by genes that govern how the cell will respond to the chemical agents. One type of multidrug resistance (or MDR) involves the ability to eject several drugs out of cells. The cell outer wall or membrane of the cell contains a pump that ejects chemicals, preventing them from reaching a toxic concentration. The resistance to drugs can be traced to the expression of genes that direct the formation of high amounts of the protein that prevents the drugs from having their effects on the malignant cells. If the gene or genes involved are not expressed or are weakly expressed, the cells are more sensitive to the drug's effect. If the genes are highly expressed, the cells are less sensitive to the drug's effect.