In everyday language, TOLERANCE implies the ability to withstand something. In pharmacology, the term tolerance is close to this meaning. To understand the technical meaning of the word, however, requires an understanding of the concept of the potency of a drug. A drug's potency is expressed in terms of the amount (the dose) of the drug needed to produce a certain effect. To illustrate, drugs may be compared with respect to potency. For example, relief from headache may be achieved with 650 milligrams of aspirin or with 325 milligrams of ibuprofen; in this case ibuprofen is said to be more potent, because less drug is needed to produce a particular effect (relief of headache). Tolerance is said to occur when a drug becomes less potent as a result of prior exposure to that drug. That is, following exposure (usually repeated or continuous administrations) to a drug, it may...