1. Ruffa RB, Friderichs E, Reimann W, Shank RP, Codd EE, Vaught JL. Opioid and non-opioid components independently contribute to the mechanism of action of tramadol, an ‘atypical’ opioid analgesic. J Pharmacol Exp Ther 1992; 260: 275–85.
2. Hennies H-H, Friderichs E, Schneider J. Receptor binding, analgesic and antitussive potency of tramadol and other selected opioids. Arzneimittelforschung 1988; 38; 877–80.
3. Houmes R-JM, Voets MA, Verkaaik A, Erdmann W, Lachmann B. Efficacy and safety of tramadol versus morphine for moderate and severe postoperative pain with special regard to respiratory depression. Anesth Analg 1992; 74: 510–4.
4. Lehmann KA, Kratzenberg U, Schroeder-Bark B, Horrichs-Haermeyer G. Postoperative patient-controlled analgesia with tramadol: analgesic efficacy and minimum effective concentrations. Clin J Pain 1990; 6: 212–20.
5. Kupers R, Callebaut V, Debois V, et al. Efficacy and safety of oral tramadol and pentazocine for postoperative pain following prolapsed intervertebral disc repair. Acta Anæsthesiol Belg 1995; 46: 31–7.