Opioid analgesics are the most frequently prescribed medications for the treatment of moderate or severe pain; however, their use is constrained by unwanted side effects. One therapeutic approach used to improve the side effect profile of opioids is the administration of a second drug in an opioid-containing mixture. Preclinical studies designed to predict the therapeutic potential of novel opioid-containing drug combinations are currently underway, and must rely on quantitative methods to assess their interactive effects. In this manuscript, an overview of isobolographic analysis is presented along with recent advances in isobolographic theory pertaining to drugs that differ in efficacy and to the statistical analysis of dose-addition. Next, studies using these analyses to assess the interactive effects of opioids and novel adjunct drugs, including selective COX-2 inhibitors, α(2)-adrenergic receptor agonists, δ-opioids, glutamate receptor antagonists and cannabinoid receptor agonists, are reviewed. Finally, comments on the future assessment of drug combinations for the treatment of pain-related disorders are made.