An adjuvant (or co-analgesic) is a drug that in its pharmacological characteristic is not necessarily primarily identified as an analgesic in nature but that has been found in clinical practice to have either an independent analgesic effect or additive analgesic properties when used with opioids.
Their therapeutic role is to increase the therapeutic index of opioids by a dose-sparing effect, add a unique analgesic action in opioid-resistant pain, or reduce opioid side effects.
A notable difference with opioids is that some adjuvant analgesics are associated with permanent organ toxicity, for example, nonsteroidal anti-inflammatory drugs (NSAIDs) and renal failure.
Most adjuvant analgesics have a ceiling effect for their analgesic actions.

Adjuvant analgesics are an essential tool in cancer pain. They form an integral part of the World Health Organization’s three-step analgesic ladder, to be used in all three steps of the ladder.

Khan MI, Walsh D, Brito-Dellan N. Opioid and adjuvant analgesics: compared and contrasted. Am J Hosp Palliat Care. 2011 Aug;28(5):378-83. Accessed on January 20, 2015.