1. The Merck Index, 11th edn., Published by Merck & Co., Inc., Rahway, NJ, 1989.
2. A. F. Casy and R. T.Parfitt, Opioid Analgesics, Plenum Press, New York, 1986.
3. A. F. Casy , Opioid Receptors and Their Ligands, in Advances in Drug Research, ed. B. Testa, vol. 18, Academic Press, London, 1989, pp. 178–272.
4. Structure-Activity Studies of Fentanyl