Local anesthesia is an indispensable tool in dentistry to minimize the acute pain and discomfort of a myriad of dental procedures.
Opioids, both illicit and prescribed, are a class of drug used for chronic and severe acute pain in an outpatient setting. Approximately 14.2% of the population is reported to use narcotics either for pain management or recreational purposes in Canada.1
While both local anesthetics and opioids target pain control, the latter may reduce the effectiveness of the former.2 It is therefore prudent to gather a thorough medical history and inquire about analgesic use, including type, duration of use, and dosage. This is especially true if a chronic pain condition is reported. Tylenol No.3®, Percocet®, codeine, morphine, hydrocodone, oxycodone are among the most prescribed opioids.3
An ex vivo study performed in 2016 found that morphine-exposed rats had a higher tolerance to lidocaine blocks than opioid-naïve rats.4 This study examined the effects of morphine on lidocaine potency and recovery from loss of lidocaine potency after cessation of morphine. Rats were exposed to varying dosages and frequencies of morphine injections; their nerves were then harvested and infused with lidocaine, and the action potentials of the nerves were measured.
To test whether tolerance to lidocaine is morphine dose-dependent, adult rats were divided into three groups and given 3, 5 or 10 mg/kg of morphine subcutaneously seven times a day. Opioid-naïve nerves were found to have lower EC50 (half maximal concentration) compared to the opioid-exposed nerves. In other words, rats that did not have previous exposure to opioids required lower lidocaine doses to block pain. When the rats were stratified by sex, the nerves from male rats demonstrated a reduction in lidocaine potency in a morphine dose-dependent manner.
Recovery from loss of lidocaine potency was assessed 7, 21, and 35 days after the cessation of 10 mg/kg morphine injections. It was observed that the nerves had decreased lidocaine sensitivity at all three time points when compared to opioid-naïve nerves. Recovery of the nerves was not evident.
Loss of lidocaine potency due to opioid use is supported by a separate study conducted in 2014 on human subjects via digital block. The mean lidocaine dose required for the same anesthetic effect was 6.67 ± 1.21 mL in opioid abusers versus 4.07 ± 1.26 mL in non-abusers.5 What’s more, the onset of local anesthesia was also found to be prolonged in narcotics users. Pain relief after the injection of lidocaine was achieved 10.30 ± 1.79 minutes in opioid users compared to 5.42 ± 1.26 minutes in non-users. The physiological process of this phenomenon is not well elucidated, and further research must be done to understand the true interaction between opioid tolerance and lowered local anesthetic efficacy.
These findings may have profound clinical implications on everyday dental practice. It suggests that dentists must prepare for the delayed onset of numbness and the possible need for increased local anesthetic doses for current or past opioid-users. Treating patients with concurrent opioid prescriptions for chronic pain or patients with opioid misuse disorder may necessitate the need for advanced anesthetic techniques or sedation to achieve sufficient anesthesia. 
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References
- Canadian Centre on Substance Use and Addiction. (2022). Prescription opioids: Canadian drug summary. Retrieved November 26, 2024, from https://www.ccsa.ca/sites/default/files/2022-11/CCSA-Canadian-Drug-Summary–Opioids-2022-en.pdf
- Brennan, T. J., Lennertz, R. C., & Kang, S. (2016). Are Opioid-tolerant Patients Resistant to Local Anesthetic Nerve Blockade?: We Need More Information. Anesthesiology (Philadelphia), 125(4), 625–626. https://doi.org/10.1097/ALN.0000000000001240
- National Institute on Drug Abuse. (2023, April). Prescription Opioids Drug Facts. Retrieved November 26, 2024, from https://nida.nih.gov/publications/drugfacts/prescription-opioids
- Liu, Q., & Gold, M. S. (2016). Opioid-induced Loss of Local Anesthetic Potency in the Rat Sciatic Nerve. Anesthesiology (Philadelphia), 125(4), 755–764. https://doi.org/10.1097/ALN.0000000000001239
- Hashemian, A. M., Omraninava, A., Kakhki, A. D., Sharifi, M. D., Ahmadi, K., Masoumi, B., & Mehrpour, O. (2014). Effectiveness of local anesthesia with lidocaine in chronic opium abusers. Journal of emergencies, trauma, and shock, 7(4), 301–304. https://doi.org/10.4103/0974-2700.142765.
About the author
Dr. Stacy Kan is a first-year dental anaesthesia resident at the University of Toronto. She obtained her DMD from McGill University and can be reached at stacy.kan@mail.utoronto.ca.
