Tolerance to a drug means that a person's expected response to the drug decreases as the drug is used repetitively so that larger doses are required to achieve the same effect. A individual will then need to take more and more of the drug to reach the high (euphoria) that he/she is seeking or to achieve the desired clinical effect (such as pain reduction with use of an opioid). Examples of drugs that often induce tolerance include opiates, benzodiazepines and alcohol. Generally speaking, tolerance is reversible. The rate of reversal depends on the drug used, the dosage of the drug and frequency of use. Reverse tolerance occurs when there is a reversal of the side effects from the drug with reduction of the insensitivity caused to a particular drug. Discontinuation of the drug to which one has developed tolerance for an extended period of time will generally induce reversal of tolerance. When the same drug (or a similar drug in the same class of drugs) is reintroduced, tolerance will occur with repeated use and may develop much more rapidly than it did initially with use of the same drug. The term cross-tolerance means that if an individual has developed tolerance to a particular drug, he may exhibit tolerance to many of the other drugs in that same drug class. For instance, a person who is developed tolerance to the opiate hydrocodone may also demonstrate tolerance to morphine. Other common drug classes that can exhibit cross tolerance include amphetamines, stimulants, benzodiazepines, and hallucinogens.

http://en.wikipedia.org/wiki/Drug_tolerance Example of cross-tolerance

http://www.biomedcentral.com/1471-2202/9/99

http://books.google.com/books?id=VgetLfJBQv0C&pg=PA103&lpg=PA103&dq=cross+tolerance+drugs&source=bl&ots=EURxpawZRe&sig=zRvW2sqBMQDWAhkJLSbgoHgNJLA&hl=en&ei=gK83SuUeyam2B6f5yNIM&sa=X&oi=book_result&ct=result&resnum=1#PPA55,M1

http://www.steadyhealth.com/encyclopedia/Reverse_tolerance