Substance abuse is very uncommon among patients with cancer. The number of known patients with cancer who are substance abusers may be small because these patients do not seek medical help in hospitals, or they may not acknowledge to health care providers that they have a substance abuse problem.
Physical dependence is defined as the occurrence of withdrawal symptoms when a drug is abruptly stopped, the dose is significantly reduced, or when a second drug is given that counteracts the actions of the drug to which the patient has developed a dependence. The dependence is not apparent until one of these actions occurs. When a patient with cancer is receiving an opioid drug for cancer pain, care is taken to avoid stopping the drug abruptly or prescribing other drugs that decrease or negate the effect of the opioid. Physical dependence on opioid pain medications does not seem to occur in patients with cancer. In these patients, once the pain disappears (usually through the effective treatment of the cancer), the pain medicine can be stopped without difficulty.
Tolerance to opioid pain medications may develop. Tolerance is the need to take increasingly larger doses of medication to relieve pain symptoms. Among patients taking opioid drugs for medical reasons, tolerance has not been shown to lead to drug addiction or drug abuse problems.
Substance abuse is the use of a drug in any manner that does not conform to the physician's orders or the use of any illegal drug.
Addiction is the use of a substance in a manner that is out of control, compulsive, used in increasing amounts, and is continued despite the risk of harm. A patient who uses opioids to relieve cancer pain may become physically dependent on the drugs, but is not described as being addicted to them.
These terms are generally used in association with people who do not have a medical illness. The terms are not entirely appropriate to use to describe medically ill people who are using drugs therapeutically.