Opioids are still considered to be the most useful analgesics available in clinical practice. However, the clear analgesic properties of opioids are counterbalanced by very frequent side effects in acute (nausea, vomiting, somnolence) and chronic use (constipation, endocrine dysfunction, hyperalgesia). Recently, it has been shown that opioid-induced hyperalgesia can develop with brief periods of treatment, for example, in the case of high doses of fentanyl to rats.

  • Fentanyl-induced hyperalgesia 

Mechanical hyperalgesia (paw pressure test)