Management of cancer pain: challenging the evidence of the recent palliative care opioid guidelines.

Opioid therapy is indisputably the mainstay for management of cancer-related pain. However, important issues as the worldwide variability on opioid availability and accessibility, myths and misconceptions about opioid use, and healthcare professionals' lacking knowledge to prescribe opioids have been pointed by researchers, clinicians, and several health organizations. A response to improve cancer pain management was the elaboration of opioid guidelines to assist practitioners to prescribe opioids to cancer-related pain. Recent opioid guidelines were developed based on systematic assessment of evidence and they are considered one of the best resources to improve knowledge and clinical practice. However, the existent ones have showed that most recommendations for cancer pain management are based on low level of evidence, which demonstrate that more studies on opioid pain management are necessary.  Moreover, the increased prescription of opioids for chronic non-cancer pain has raised other issues as iatrogenic adverse effects, which may also occur in patients with cancer pain in long-term opioid therapy (L-TOT). In this narrative review the role of opioid guidelines and recent knowledge regarding consequences of L-TOT are discussed, in particular opioid addiction and deficiencies of the immune and endocrine systems. Finally, new strategies to strengthen L-TOT in cancer pain management in patients in palliative care have been addressed.