There is an important difference between drug dependence and drug tolerance although sometimes these terms are used interchangeably. Drug dependence occurs when the patient has a psychological and/or physical dependence on the drug that results in withdrawal symptoms that can become unbearable for the patient to endure. Relief comes only when the patient is administered a dose of the drug.
Tolerance occurs when the concentration of a drug no longer has a continuing therapeutic effect. This happens because receptor sites in the effective tissues adapt to the prescribed level of drug. The only way to regain the therapeutic effect is to increase the concentration of the drug.
Tolerance also has a metabolic effect commonly referred to as pharmacologic tolerance. Prolonged exposure to a drug increases the excretion of the drug from the body. This means there is a lower concentration of the drug in plasma that is distributed throughout the body.
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