Combining opioid and adrenergic mechanisms for chronic pain.

Chronic pain is a highly prevalent medical problem in the United States. Although opioids and serotonin-norepinephrine reuptake inhibitors (SNRIs) have demonstrated efficacy for relief of chronic pain, each has risks of adverse events in patients. Because of the risk of opioid abuse and addiction, combinations reducing opioid requirements are particularly valuable. Opioid and SNRI agents relieve pain by different pathways; concurrent use of each agent separately offers many potential benefits: complementary and possibly synergistic analgesic efficacy, separate titrations of opioid and SNRI effects, and the reduction of opioid requirements. However, few clinical studies have investigated the ideal ratios for combinations of opioids and SNRIs. A number of factors affect whether specific combinations have additive, synergistic, less than additive efficacy, or increase adverse events in patients, including general pharmacokinetic considerations, the potential for pharmacodynamic drug interactions, dose, and timing. Because there is little clinical evidence guiding combination therapy with separate opioid and SNRI agents, using single-molecule agents provides safe and effective therapy and should be the first option presented to patients. The use of empiric combinations of separate opioid and SNRI combinations needs to be considered in light of clinical cautions, including the lack of published evidence to guide dose conversion from any opioid to tramadol or to tapentadol, and vice versa; the need to avoid combinations with known drug interactions; and the need to titrate the dose when adding an SNRI to an opioid, and vice versa.