Hello and welcome to HCM,
Oxycodone is not anti cancer drug.
It is an opioid analgesic
Probably your father is suffering from extreme pain secondary to rectal cancer
This preparation is used to control pain not controllable by NSAIDs.
This is a part of palliative care
To dose of analgesic varies from patient to patient depending on the extend of disability caused by the cancerous process.
To know more about the dosage, you will have to consult your oncologist
Thanks and take care
Dr Shailja P Wahal