Inclusion Criteria:  Male and female cancer patients receiving a WHO step III opioid (i.e. morphine, oxycodone, methadone, fentanyl, hydromorphone, or buprenorphine) in combination with paracetamol (minimum dose 3 g/day) Age ≥ 18 at screening Ability to understand the study procedures and to provide written informed consent Stable analgesia before randomisation, defined as no required changes in the analgesic treatment during the previous 7 days  Exclusion Criteria:  Participation in another interventional trial within 30 days prior to randomisation, with the exception of cancer treatment trials Changes of the dosage or start of other (co-)analgesics (e.g. tricyclic antidepressants, neuroleptics, nonsteroidal anti-inflammatory drugs (NSAIDs), dipyrone), within the last 7 days preceding randomisation Surgery within the 14 days preceding randomisation or surgery planned within the following 14 days after randomisation Any circumstances, comorbidities or conditions, which, in the opinion of the investigator, may affect full participation in the study or compliance with the study protocol