2.1 Data
Reddit is particularly popular among PWUO and the broader community of people who use drugs (PWUD) as it can offer anonymity.11 Reddit has seen rapid growth in its user base over the last several years. In 2021, it is estimated that Reddit has over 430 million12 monthly active users (Redditors), surpassing the number of active Twitter users.13 These increases may be driven by factors such as the organization of discussions through specialized topical-interest forums called subreddits and the moderation of forum-specific content by forum users. Reddit communities have also been found to serve as a means of social support for PWUD.14–16
To find potential PWUO on Reddit, we identified 14 opioid-related subreddits spanning discussions on prescription and illicit opioids, and MOUD (Appendix A1) and collected all retrievable posts contained using the Python-Reddit application programming interface (API) Wrapper for Reddit.19 Since these subreddits serve as specific discussion forums for topics related to opioid use and recovery and empirical examination of posts revealed that most Redditors discussed personal use and experience, we assumed that all authors of posts that mentioned any opioid or opioid-related medications defined as opioid reversal agents and MOUD were either current or past PWUO/PTMOUD. We also assumed that mentions of a substance corresponded with presumed use of that substance. Thus, Redditors who mentioned opioids and/or opioid-related medications and stimulants were presumed to co-use both substances (PWCU) during the study period – in our analyses, we separately examined co-mentions of both substances within a given year as well as mentions that may have spanned multiple years. After retrieving subreddit posts of the 47,327 Redditors who had posted on the selected sub-reddits, we selected a random sample of these Redditors (N=13,812) and collected each of their past public posts across all subreddits (i.e., timelines), between November 2006 (corresponding to the earliest post available) and July 2021 (corresponding to the last date of data collection). Each timeline took approximately 20 minutes to obtain, so the number of Redditors included in the sample was limited by the rate at which we could collect full timelines via the API. We studied trends in opioid and stimulant co-use between January 1, 2011 and December 31, 2020. We excluded earlier years from this analysis because the numbers of posts (and correspondingly, Redditors) prior to 2011 were very low (i.e., ≤20 posts) , and we excluded 2021 because complete data for this year were not available. Due to similar concerns related to low posts with specific substances mentioned, we restricted our study of specific opioid and opioid-related medications, and stimulant categories to the years 2015 – 2020. We then chronologically ordered the posts per Redditor to enable an exploration of substance use timelines. All analyses described in this study were conducted using Python.
2.2 Identifying substance mentions
We apply natural language processing methods to improve the detection of true mentions of opioids and stimulants, and to exclude false positives (e.g., negated concepts and ambiguous expressions). We focused the study on a set of common opioids, opioid-related medications, and stimulants, including both prescription and illicit types (Appendix A2). Since drug names and related expressions are often misspelled on social media, we generated commonly used lexical variants of the terms using the LexExp tool.20 We found that some non-standard terms and lexical variants tend to have high noise associated with them (i.e., expressions not actually referring to a stimulant or opioid, e.g., oxy clean). Thus, we included additional filters for the terms stimulant, meth, and oxy (Appendix A3). We applied an additional negation detection strategy, customized for Reddit data, to exclude negated drug mentions. Specifically, we used a subset of negation terms from the NegEx algorithm21 (Appendix A4) with a moving context window, determined empirically, of size n = 5 following the mention of a negation trigger. Any opioid, opioid-related medication, or stimulant term was considered negated if it appeared in the context window following a detected negation expression and if an end of sentence marker (e.g., a period) did not occur between the negation term and the drug term. Negated terms were excluded from the final count of mentions. We also manually reviewed samples of posts detected by our searches to identify keywords with a large amount of noise (e.g., ‘dope’ for heroin and ‘coke’ for cocaine) and excluded those from our counts and further analysis. Of the 13,812 Redditors sampled, a subset who posted in these subreddits had no mention of either an opioid, opioid-related medication, or a stimulant (N=1,506) and were excluded from the analyses. All analyses except for those examining the total numbers of redditors (including PWCU) over time were based on subsets of the remaining 12,306 Redditors who posted about at least one opioid, opioid-related medication, or stimulant.
2.3 Opioid and stimulant co-use trends
We first studied trends in opioid and stimulant co-use between 2011-2020 through multiple metrics, including, the number of Redditors posting each year; the number of Redditors who mentioned at least one opioid or opioid-related medication and one stimulant (i.e., PWCU) over the same time frame; and the annual ratio of PWCU to PWUO/PTMOUD . Due to the growing user-base of Reddit over time, we expected numbers of PWUO/PTMOUD and PWCU to both increase annually. Hence, the ratio of PWCU to PWUO/PTMOUD likely serves as the best indicator of social media trends in opioid and stimulant co-use within this population.
Next, we calculated and visualized three main explorations of co-use patterns for our dataset, focusing on the years 2015-2020 due to lower counts of posts by specific substances in prior years, – i) frequency of co-use of specific types of opioid-stimulant pairs among PWUO/PTMOUD aggregated over the entire study period; ii) proportion of specific opioids co-used with any stimulant in a given year among the total number of PWCU in that year, to study how mentions of specific opioids or opioid-related medications co-used with any stimulant changed over time; iii) proportion of specific stimulants co-used with any opioid or opioid-related medication in a given year among the total number of PWCU in that year, to study how mentions of specific stimulants co-used with any opioid changed over time. Redditors with mentions of more than one type of opioid or opioid-related medication or stimulant within a given year were separately considered for each specific opioid-stimulant pair.
We combined the opioids and opioid-related medications listed in our analytic sample into 5 categories: (i) heroin, (ii) fentanyl & analogs (including carfentanil), (iii) Medications for Opioid Use Disorder (MOUD; e.g., buprenorphine, methadone, naltrexone), (iv) opioid overdose reversal agents (e.g., naloxone), and (v) prescription opioid pain relievers (e.g., oxycodone, hydrocodone, and tramadol). We combined stimulants into 4 categories: (i) methamphetamine, (ii) amphetamine-type stimulants (e.g., Adderall®, dextroamphetamine, levoamphetamine), (iii) methylphenidate-type stimulants (e.g., Ritalin®), and (iv) a combination of methamphetamine and heroin (i.e., goofball/speedball). Categories included generic names, trade names, and common misspellings or lexical variants (Appendix A2). Due to aforementioned challenges of appropriately identifying keywords with substantial noise, we excluded mentions of cocaine from our analysis.
2.4 Timeline analyses
One of our objectives was to explore when stimulants are initiated and used by PWUO/PTMOUD over time. We focused specifically on methamphetamine initiation and use as our analyses revealed methamphetamine to be the most commonly used stimulant by a large margin (Appendix A5). We constructed a cohort by selecting the subset of Redditors who mentioned an opioid or opioid-related medication first in their timelines of posts from our randomly selected sample. To align the timelines of different Redditors, we considered the date of the first opioid mention to be Day 0. Timelines were stratified based on opioid type by grouping together Redditors who mentioned the same or similar opioids on Day 0. We tracked timelines for each PWUO/PTMOUD by computing the monthly frequencies of methamphetamine mentions for 24 months starting from Day 0. We excluded Redditors whose first and last post on Reddit were less than 24 months apart and those who had any stimulant-related posts within 30 days of their first opioid or opioid-related medication post (i.e., individuals with presumed co-use within the first month). By mapping the first post to Day 0 for all Redditors who posted, the distribution of posts over the 24 months was naturally skewed— each Redditor had at least one post in the first month, but the frequency of posts over the following months varied. To adjust for this, we normalized the monthly methamphetamine mention frequencies by the total posts made by PWUO/PTMOUD each month. Since most mentions correspond to personal use, we assumed Redditors with an opioid or opioid-related medication mention first and subsequent methamphetamine mention had transitioned to co-use. We developed heatmaps of these timelines to visualize temporal patterns of methamphetamine use.