Tolerance is a normal physiological response to chronic opioid therapy in which increasing doses are required to produce the same effect.
Experience with cancer patients with chronic pain indicates that significant tolerance is uncommon and the need for increasing doses usually relates to disease progression. Concern regarding tolerance is not a reason for "saving up" the use of opioid drugs until the terminal phase. Patients concerned that there will be "nothing left" for more severe pain should be reassured that the therapeutic range of morphine is very broad and there is adequate scope to treat more severe pain if it occurs.
Cross-tolerance between the various opioid drugs is not complete and an alternative drug can be substituted if the rate of development of tolerance is of concern.

Woodruff R. Palliative medicine evidence-based symptomatic and supportive care for patients with advanced cancer. 4th ed. Oxford University Press, 2004. p. 105.

Analgesic tolerance refers to the diminishing analgesic effect of an opioid because of physiological adaptation to the opioid. Analgesic tolerance is not inevitable, but it does occur. The dose of an opioid such as morphine typically can be increased to overcome any tolerance that may develop, as long as side effects are tolerated. Analgesic tolerance should not be confused with the need to increase dose because of an increase in pain.

University of Wisconsin-Madison Pain & Policy Studies Group. Glossary of Terms. Internet. Accessed March 9, 2017.