Tolerance is the ability of a person's physiology to adapt to the presence of a pharmaceutical, illegal drug or even a toxin so that it takes a larger dose of the drug to have the same effect.  It is often why addicts stop feeling the pleasant sensation of a drug even though they take a near-fatal dose, while at the same time being able to take doses that would result in an overdose for a non-user.

In medicine, many pharmaceuticals become tolerated and cease to have a therapeutic effect, even at increasingly higher doses of the drug.  This should not be confused with resistance, where the disease becomes increasingly resistant to the drug.  

For example, the use of morphine acutally creates more morphine receptors in the body.  After a while, the dose of morphine necessary becomes toxic and becomes ineffective in treating pain.

Drug tolerance at Wikipedia

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