Physical dependence is a normal physiological response to chronic opioid therapy, which leads to withdrawal symptoms if the drug is abruptly stopped or an antagonist administered.
The withdrawal syndrome is characterized by anxiety and agitation, rhinorrhea and lacrimation, fever and sweating, muscular and abdominal cramps, tremor and other signs of sympathetic overactivity.
The time of onset and duration of the withdrawal syndrome depend on the pharmacokinetics of the opioid in use. Withdrawal syndromes are treated by administering 25-50% of the previous daily dose, given in divided doses, 6-hourly.
Patients whose pain has been relieved by surgical or other means should have their opioid analgesics reduced by 15-25% per day.
Patients need to be reassured that physical dependence does not prevent withdrawal of the medication if their pain is relieved by other means.

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