Cancer Pain and Co-Analgesics

Introduction — cancer pain & the co-analgesic rationale

• Cancer pain is multi-mechanistic — clinical cancer-pain syndromes typically combine nociceptive/inflammatory and neuropathic mechanisms rather than a single pain type; pain is treated as the "fifth vital sign" and adequate assessment/relief is a standard-of-care and legal obligation in many jurisdictions.
• Two dimensions of pain — the sensory-discriminative (intensity) and affective-motivational (suffering) dimensions are pharmacologically distinct — opioids act on both, and even sub-analgesic morphine can blunt the affective response before perceived intensity.
• Mechanism dictates drug choice — continuous dull inflammatory pain is relieved well by opioids; neuropathic pain responds less well to opioids (needs higher doses) and not at all to NSAIDs — the pharmacological rationale for recruiting co-analgesics.
• Co-analgesic (adjuvant analgesic) — definition — a drug whose primary indication is not pain but which "enhances analgesic efficacy, treats concurrent symptoms that exacerbate pain, or provides independent analgesic activity for specific types of pain" — usable at every step of the ladder (examples: antidepressants, anticonvulsants, corticosteroids, bisphosphonates, ketamine, topical agents).