Doctor of Philosophy
James P. McCullough
Introduction. While opioid medications are commonly prescribed for management of neuropathic pain (NP), long-term use has been associated with increased risk for overdose, drug interactions and addiction. New strategies are necessary to better manage chronic pain, thereby reducing need for opioid medications and their associated adverse consequences. N-acetyl-L-cysteine (NAC), an over-the-counter supplement, has shown promise in the treatment of psychiatric and addictive disorders. In addition, NAC has shown promise for reducing physiological signs of NP in laboratory rat models, prompting this study.
Purpose. The present study was an open-label clinical trial of NAC as an adjuvant to opioid treatment for poorly controlled, chronic NP. It examined whether 1200 mg NAC twice daily for 4 weeks was associated with: lower ratings of patient-reported pain; reductions in PRN opioid medication for breakthrough pain; and improvements in physical and mental health quality of life (QoL). The study also examined whether appraisal of pain impacts response to medication.
Method. Participants were N=28 chronic NP patients who consented to study participation. This consisted of 2 baseline assessments, 4 weeks of NAC and 1 post-trial follow-up visit. The majority (N=17) dropped out or were excluded during baseline. Of the remaining participants, N = 11 started the study medication and N=10 completed the study, with daily recordings of pain severity ratings and use of PRN opioid medication. Small sample size limited analyses to qualitative case reviews and effect sizes.
Results. Over 90% of participants receiving NAC completed the study. Case review found varied results. While 4 of 10 participants showed decrease in average pain ratings during NAC, estimated effect sizes for the whole sample were small, bordering on negligible (ω² from .003 to .027) as were those for PRN opioids (Partial Eta-Squared=.0003). Effect size for mental health QoL was medium (Cohen's d=.421).
Conclusions. With N=10, findings must be interpreted with caution. Nonetheless, the study found some albeit small evidence supporting NAC for improving mental health QoL and pain ratings. Several participants reported improvements in pain and mental health domains while taking NAC. NAC was well tolerated with minimal side effects. Lessons from this study will inform design and implementation of future NAC studies.
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