Introduction

Chronic pain affects approximately one-third of the U.S. population, and opioid prescriptions have substantially increased over the last 20 years [1]. In parallel, there has been an increase in opioid-related complications, with opioid overdose deaths quadrupling between 1999 and 2015 [2]. Growing concerns about the risks of opioids, including overdose-related deaths and opioid use disorder, have prompted greater focus on the more judicious use of these agents for managing pain and the need to identify other agents to treat pain [3].

The data on the efficacy of cannabinoids in the management of pain is evolving. In a systematic review, there was low-strength evidence that cannabis is effective for treating neuropathic pain and insufficient evidence of its effectiveness for other types of pain [4]. The American Academy of Neurology has endorsed use of cannabinoids for the pain and spasticity associated with multiple sclerosis but cautions that the safety profile of cannabinoids has not been compared to other approved drugs [5]. Despite the lack of robust evidence for efficacy of cannabinoids in pain management, marijuana has been approved by legislatures or ballot initiative for the management of pain in over 30 states [6].

Recent data suggest that medical marijuana laws have been associated with lower state-level opioid overdose mortality, hospitalizations related to opioid complications, detection of opioids among fatally injured drivers, and prescription of analgesics [7]. These ecologic studies, while hypothesis generating, do not inform our understanding of the individual effects of marijuana use or combined marijuana and opioid use. Prospective cohort studies and clinical trials are needed to improve our understanding of the effects of cannabis on pain management. Nonetheless, these studies have spurred discussion about the potential for marijuana to serve as a substitute for opioids, particularly in contexts where marijuana is increasingly available through legalization. Small surveys of convenience samples of American and Canadian marijuana users have reported that substitution of marijuana for opioids is common, ranging from approximately 30% to 97% [8, 9]. To our knowledge, there are no nationally representative surveys examining substitution and reasons for substitution among the general US adult population. We examined the prevalence and reasons for substitution of marijuana for opioids among US adults taking opioids for pain, as well as the factors associated with substitution.

Methods

Results

There were 9,003 respondents, corresponding to a 55.3% response rate. Baseline characteristics of respondents were similar to respondents from the National Survey on Drug Abuse and Health, though our respondents had a slightly higher average income, (Table A in S1 File) suggesting our sample was representative of the US population [10]. The mean age was 48 years, 48% were male, 64% were white, and 64% lived in a state in which marijuana was legal. Among this national sample, forty-six percent reported ever using marijuana, and 8% reported regular use of opioids for pain in the past year.

Among the 5% (n = 486) who reported ever using marijuana and using opioids in the past year, 43% used opioids daily, and 23% reported current (past 30 day) marijuana use (Table 1). Forty-one percent reported a decrease or cessation of opioid use due to marijuana use; 46% reported no change in opioid use; and 8% reported an increase in opioid use. The most commonly reported reasons for substitution were better pain management (36%) and fewer side effects (32%) and withdrawal symptoms (26%), compared to the non-medical reasons for use: cheaper (13%) and more social acceptance from marijuana use (13%). In multivariable analyses, we found no association between socio-demographics or status of marijuana legalization in the state of residence and substitution (Table 2).

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Table 1. Frequency of opioid and marijuana use, prevalence of substitution of marijuana for opioids, and reasons for substitution among persons reporting marijuana and opioid use in a national survey of US adults conducted in fall 2017.

https://doi.org/10.1371/journal.pone.0222577.t001

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Table 2. Characteristics associated with substitution of marijuana for opioids.

https://doi.org/10.1371/journal.pone.0222577.t002

Discussion

In a nationally representative survey of US adults, substitution of marijuana for opioids, which included a substantial degree of opioid discontinuation (~20%), was common. Better self-reported pain management and fewer side effects and withdrawal symptoms were the most common reasons for substitution. Our findings are consistent with prior surveys of American and Canadian marijuana users in which substitution of marijuana for opioids was prevalent due to better symptom management and fewer adverse and withdrawal effects [8].

Our study overcomes the potentially biased reporting in favor of substitution from prior convenience samples of marijuana users. This may explain why the prevalence of substitution in our study was lower than that of other studies in which a prevalence of up to 97% has been reported [8]. Additionally, we focused specifically on substitution of marijuana for opioids and asked about this practice directly whereas other studies asked about substitution of marijuana for prescription drugs more broadly [18–20] or indirectly assessed opioid substitution [21]. Our results were also inconsistent with a recently published Australian cohort study which followed approximately 1,500 people with chronic non-cancer pain prescribed opioids for four years [22]. About a third of participants reported that they sometimes or regularly reduced their opioid medication when using cannabis. However, the prevalence of opioid discontinuation was not significantly different between daily or near-daily marijuana users and non-users [22]. To date, data on self-reported improvement of symptoms has not been substantiated by studies that have monitored opioid and marijuana use. More research on this topic is clearly needed. Nonetheless, our findings suggest that even if objective measures do not support that marijuana is substitutive for opioid use, patients perceive that marijuana use has reduced their opioid use. Perhaps the commercialization of marijuana and the favorable media coverage surrounding the health effects of marijuana are fostering such a perception [23, 24]. More research including clinical trials on the efficacy of cannabis in pain management with the inclusion of patient-centered outcomes is needed to shed light on the role of marijuana on pain management.

Our study has several limitations that deserve comment. Our study was a cross-sectional survey of a relatively small number of respondents with a history of marijuana use and opioid use within the past year. Our survey question provided a limited number of examples of opioid medications (i.e., Vicodin, Percocet, or OxyContin), so it is possible that we did not identify all opioid users. Although our survey specifically asked about the use of opioids for pain, it is possible that we captured individuals who were using opioids for other reasons such as opioid use disorder. Likewise, it is possible that respondents may be using marijuana for reasons other than pain. Thus, we cannot conclude with certainty that patients are using marijuana as an alternative to opioids for pain per se. We also relied on respondents’ retrospective judgment regarding reduction or cessation of opioid use attributable to marijuana use. Thus, our findings could also be limited by recall bias, although this was minimized by restricting the sample to those who used opioids within the past year. Underreporting of substitution could have occurred, particularly in states in which marijuana has not been legalized.

Conclusions

In conclusion, our study provides further evidence that patients self-report that they are substituting marijuana for opioids. The impact of marijuana substitution for opioids and concomitant opioid and marijuana use on health outcomes is unknown and warrants investigation.

Supporting information

Acknowledgments

Disclaimer: The views expressed in this article are those of the authors and do not represent the views of the VA or the US government.

The funding organizations had no role in the design of the study; collection, analysis, and interpretation of the data; or the decision to approve publication of the finished manuscript.

The authors had access to all the study data, take responsibility for the accuracy of the analysis, and had authority over manuscript preparation and the decision to submit the manuscript for publication.