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. The therapeutic role of adjuvant analgesics (AAs) 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 between opioids and AAs is that unlike opioids some AAs are associated with permanent organ toxicity, for example, nonsteroidal anti-inflammatory drugs (NSAIDs) and renal failure. It is impossible to predict in advance in a given individual what opioid dose they may require to control cancer pain. Most AAs have a ceiling effect for their analgesic actions, but often with continued dose-related toxicities and side effects (with the exception of glucocorticoids). The blood levels of opioids (and their metabolites) can be measured with great precision and accuracy. There is sometimes a role for drug blood levels of certain AAs, like tricyclic antidepressants or anticonvulsants when used for neuropathic pain. Age affects metabolism of most opioids. The therapeutic window of opioids is wide, with no ceiling effect. Most AAs (except corticosteroids) have a narrow therapeutic window. Naloxone is a pure opioid antagonist that competes and displaces opioids from their receptor sites. All clinically useful opioids are mu opioid receptor agonists. Not all routes of administration are available to all opioids. Adjuvant analgesics lack the versatility in routes of administration that opioids possess. Dosing flexibility is a major advantage when treating cancer-related pain with opioids. Dose flexibility is much less with AAs than opioids. Unlike opioids, the analgesic response is usually observed within hours to days of attaining an adequate dose with most AAs (1-2 days). Rotation among opioids is a useful therapeutic strategy to improve analgesic response or minimize toxicity. Most AAs are unsuitable for rescue dosing because of their pharmacological characteristics. The mu agonist side effect profile is similar among the different opioid agents, regardless of the route of administration. The appropriate use of AAs will reduce opioid-related side effects. No apparent tolerance to analgesia develops with AAs. Abrupt discontinuation of an opioid after chronic repeated use for more than a few days will cause a withdrawal syndrome of variable severity. Adjuvant analgesics are an essential tool in cancer pain.