Tolerance and Resistance
Tolerance is a person's diminished response to a drug, which occurs when the drug is used repeatedly and the body adapts to the continued presence of the drug. Resistance refers to the ability of microorganisms or cancer cells to withstand the effects of a drug usually effective against them.
A person may develop tolerance to a drug when the drug is used repeatedly. For instance, when morphine or alcohol is used for a long time, larger and larger doses must be taken to produce the same effect. Usually, tolerance develops because metabolism of the drug speeds up (often because the liver enzymes involved in metabolizing drugs become more active) and because the number of sites (cell receptors) that the drug attaches to or the strength of the bond (affinity) between the receptor and drug decreases (see Section 2, Chapter 12).
Strains of microorganisms (bacteria or viruses) are said to develop resistance when they are no longer killed or inhibited by the drugs that are usually effective against them. In bacteria, these strains develop when bacteria mutate or when resistant genes are transferred from one bacterium to another (see Section 17, Chapter 192). Cancer cells may develop resistance when they mutate.
Depending on the degree of tolerance or resistance that develops, doctors may increase the dose or use a different drug.
| 
