Combination opioid analgesics.

Although there is no "ideal analgesic," scientists and clinicians alike continue to search for compounds with qualities which may approach the "ideal analgesic." Characteristics of an "ideal" analgesic may include: the agent is a full agonist providing optimal/maximal analgesia for a wide range/variety of pain states (e.g., broad spectrum analgesic activity), it does not exhibit tolerance, it produces no unwanted effects and minimal adverse effects, it has no addictive potential, it does not facilitate pain/hyperalgesia, it has a long duration, it has high oral bioavailability, it is not vulnerable to important drug interactions, it is not significantly bound to plasma proteins, it has no active metabolites, it has linear kinetics, and it is eliminated partly by hydrolysis to an inactive metabolite (without involvement of oxidative and conjugative enzymes). Investigators have concentrated on ways to alter existing analgesics or to combine existing analgesic compounds with compounds which may improve efficacy over time or minimize adverse effects. The addition of an analgesic with a second agent (which may or may not also be an analgesic) to achieve a "combination analgesic" is a concept which has been exploited for many years. Although there may be many reasons to add 2 agents together in efforts to achieve analgesia, for purposes of this article - reasons for combining an opioid with a second agent to produce a combination opioid analgesic may be classified into 6 major categories: 1.) combinations to prolong analgesic duration; 2.) combinations to enhance or optimize analgesic efficacy (e.g., analgesic synergy); 3.) combinations to diminish or minimize adverse effects; 4.) combinations to diminish opioid effects which are not beneficial (or contrariwise to or enhance beneficial opioid effects); 5.) combinations to reduce opioid tolerance/opioid-induced hyperalgesia; and 6.) combinations to combat dependency issues/addiction potential/craving sensations. Combination opioid analgesics are one avenue which may give rise to "pain pills" with improved analgesic profiles over existing analgesic medications.