| Level I | Good quality evidence |
Systematic review of RCTs with consistent findings High quality individual RCT |
| Level II |
Limited quality patient orientated evidence |
Systematic review of lower quality studies or studies with inconsistent findings Low quality clinical trial Cohort studies Case-control studies |
| Level III | Other |
Consensus guidelines, extrapolations from bench research, usual practice, opinion, disease-oriented evidence (intermediate or physiologic outcomes only), or case series |
[6] Methadone, oral: Dietis N, Guerrini R, Calo G, Salvadori S, Rowbotham DJ, Lambert DG. Simultaneous targeting of multiple opioid receptors: a strategy to improve side-effect profile. Br J Anaesth 2009; 103: 38–49.
http://www.ncbi.nlm.nih.gov/pubmed/19474215
Methadone, oral: Holtman JR, Jr, Wala EP. Characterization of the antinociceptive and pronociceptive effects of methadone in rats. Anesthesiology 2007; 106: 563–71. http://www.ncbi.nlm.nih.gov/pubmed/17325516