Pain is an unpleasant sensory and emotional experience associated with actual or potential tissue damage, which is often referred to as acute or chronic. Acute pain serves a vital function – it makes us aware of damage to the body and makes us behave accordingly: in other words, we seek treatment.

Chronic pain (i.e. pain that persists) can be classified as:

• Peripheral – originates from outside the central nervous system (CNS)
• Neuronal – a disturbance in the pain processing mechanism in the brain
• Mixed – a peripheral mechanism that triggers central mechanisms in the brain.

Cancer pain tends to be peripheral in origin since it is caused primarily by damage to surrounding intact tissue from excessive tissue growth (metastases).