Background: Control of chronic pain and mainly the partial or complete loss of response to
analgesics is a major unmet need. Multiple mechanisms underline the development of tolerance to
analgesics in general and specifically to opioids. The autonomic nervous system (ANS) plays a role
in the development of analgesic tolerance and chronobiology.
Objectives: To review the mechanisms associated with the development of nonresponsiveness
to analgesics.
Study Design: Literature review.
Setting: The review is followed by a description of a new method for overcoming resistance and
improving the response to analgesics.
Methods: Conducted a detailed review of the relevant studies describing the mechanisms that
underlie tolerance to pain medications, and the potential roles of the ANS and chronobiology in
the development of drug resistance.
Results: The autonomic balance is reflected by heart rate variability, an example of a fundamental
variability that characterizes biological systems. Chronotherapy, which is based on the circadian
rhythm, can improve the efficacy and reduce the toxicity of chronic medications. In this article,
we present the establishment of an individualized variability- and chronobiology-based therapy
for overcoming the compensatory mechanisms associated with a loss of response to analgesics.
We describe the premise of implementing personalized signatures associated with the ANS, and
chronobiology, as well as with the pathophysiology of pain for establishing an adaptive model that
could improve the efficacy of opioids, in a highly dynamic system.
Limitations: The studies presented were selected based on their relevance to the subject.
Conclusions: The described variability-based system may ensure prolonged effects of analgesics
while reducing the toxicity associated with increasing dosages.
Key words: Painkillers, opioids, drug resistance, compensatory mechanisms