www.iasp-pain.org/AM/Template.cfm?Section=Pain_Definitions
This is an excellent International Association for the Study of Pain (IASP) resource for nurses to better understand the evidence behind effective pain management. 

www.who.int/cancer/palliative/painladder/en 
This web page will give you a better understanding of the Pain Ladder; it was developed by WHO for cancer pain relief.

www.nmc-uk.org/Documents/NMC-Publications/NMC-Standards-for-medicines-management.pdf 
The NMC standards for medicine management, required by all nursing staff.

www.evidence.nhs.uk/search?q=Acute%20Pain%20Guidelines 
The main portal to review the evidence supporting methods and guidelines to enhance pain management within a variety of conditions.