The Faculty of Pain Medicine (Royal College of Anaesthetists) states that the dose above which harms outweigh benefits is 120mg oral morphine or equivalent/24hours.
Above this dose the risk of harm and mortality increases substantially but there is no increased benefit.
Increasing the dose of opioids may lead to worsened function, muscle rigidity, hypotension, respiratory depression, decreased immunity, decreased libido, increased anxiety, increased pain sensitivity and increase the risk of dose dependent adverse effects.
