Detecting Misconception Surges in Opioid Policy: A Zero-Shot LLM Analysis of British Columbia’s Decriminalization Pilot

Background: In January 2023, British Columbia (BC) launched Canada's first drug decriminalization pilot in response to persistently high opioid-related deaths. This initiative removed criminal penalties for possession of up to 2.5 grams of certain illicit drugs to reduce stigma and increase service use. Social media platforms such as Reddit provide a real-time window into public perceptions and misconceptions about such policies. Methods: We collected 22,131 comments posted between January 2023 and October 2024 from subreddits discussing decriminalization in British Columbia. A zero-shot large language model applied a multi-label taxonomy with four misconception classes and one no-claim class. Expert adjudication of 547 comments with model disagreement supported selection of the primary classifier for analysis. We tracked monthly misconception prevalence and used subreddit-week reply premiums and lattice clustering to detect engagement bursts. Negative binomial models quantified reply differences across periods, regions, and comment-level harm reduction references. Results: GPT-4o labeled 4,473 comments as containing at least one misconception, and policy-target misinterpretation and impact claims dominated through early 2024. After the May 2024 amendment that restricted possession in public spaces, Legal-Status Confusion and Enforcement Misconceptions increased and peaked in June 2024. These comments received an 11% reply premium, with an incidence rate ratio of 1.11 (95% CI 1.02-1.21), and the premium increased after May 2024 (1.18, 1.16-1.21). Reply premiums clustered into four bursts from February to July 2024, and each burst lasted 2 to 5 weeks across 2 to 5 subreddits. Harm-reduction terms predicted higher replies, with an incidence rate ratio of 1.12 (95% CI 1.04-1.20), and the interaction ratio was 0.81 (0.66-0.98). Community-level harm-reduction visibility did not predict week-to-week variation in reply premiums for either misconception class. Conclusions: Public discussion of BC's decriminalization policy revealed persistent confusion, and legal and enforcement confusion intensified around the May 2024 revision. Burst detection on social platforms can identify when and where confusion concentrates, which supports timely public clarification and service signposting. During future amendments, concise messages about permissible locations and remaining police authority may reduce avoidable enforcement contact and misinformation.