Bush SH. Textbook of Palliative Medicine. London, Hodder Arnold, 2006. pp. 505-511.
Analgesics. Twycross R, Wilcock A (Ed). Palliative Care Formulary. Third Edition, Palliativedrugs.com Ltd, United Kingdom. 2007, pp.209-328.
Episodic pain. IAHPC Pallipedia. https://pallipedia.org/episodic-pain/apple-touch-icon-152x152.png. Accessed