Pupil Response in Patients on Opioids.

Inclusion Criteria: * In-patients and out-patients at Oslo University Hospital with cancer pain who have use opioids (morphine, oxycodone, fentanyl or methadone or a combination of these drugs) at least for periods of 4 weeks, and use opioids corresponding to at least 60 mg oral morphine equivalents pr day Exclusion Criteria: * Patients who have undergone eye-surgery that may influence pupillary reflexes * Patients on local medication that may influence pupillary reflexes * Patients with amyloidosis, multiple sclerosis, Horner's syndrome, or ongoing migraine attack * Patients with brain tumor