2.1. Search strategy and selection criteria
According to PRISMA Extension Statement for Reporting of Systematic Reviews Incorporating Network Meta-analyses[14], MEDLINE, Embase, The Cochrane Central Register of Controlled Trials and Web of Science were searched with language restrictions in English. The search strategy combined free text words and MeSH terms to maximize the results yielded. Search terms were used as follows: (1) opioid, (2) hyperalgesia and (3) magnesium, naloxone, buprenorphine, ketamine, dexmedetomidine, butorphanol, propofol, flurbiprofen, morphine, methadone, lornoxicam, nitrous oxide, parecoxib, clonidine, amantadine, nalbuphine, paracetamol, pregabalin, nefopam, acetazolamide.