One of the prevailing views about opioid use is that it is common for persons using the drug for medical purposes to become addicted. While this does occur, it is not common among those who legitimately need the drug for pain and who have no history of substance abuse or addiction. Addiction is sometimes confused with tolerance in cases where patients are receiving opioids for serious pain. Tolerance to opioids happens when the drug dose has to be increased to produce a similar pharmacological effect. When tolerance develops, there is also a decrease in the length of time that the opioid produces its effect. In patients with intractable cancer pain, doses can usually be increased carefully over time without serious risk to the patient.