Trends in Hallucinogen-associated Emergency Department Visits and Hospitalizations in California from 2016–2021

Objectives Hallucinogens encompass a diverse range of compounds with increasing scientific and public interest. Risks associated with hallucinogen use are poorly understood. This study aims to evaluate the trends in hallucinogen-related emergency department visits and hospitalizations in California from 2016 to 2021, as compared to alcohol and cannabis visits and hospitalizations Methods We conducted a descriptive study on publicly available data on diagnosis codes associated with emergency department (ED) visits and hospitalizations from the California Department of Healthcare Access and Information (HCAI) to describe hallucinogen, alcohol, and cannabis associated encounters. Results Rates of hallucinogen associated ED visits increased by 69% between 2016 and 2021 compared to a 24% decrease and 1.9% decrease in rates of alcohol associated and cannabis associated ED visits respectively. Rates of hallucinogen associated hospitalizations increased by 74% compared to a 11% increase in alcohol associated hospitalizations and a 19% increase in cannabis associated hospitalizations. Conclusions The large relative, but small absolute, increase in hallucinogen associated ED visits and hospitalizations is concerning given changing public perception and increasing use of these substances. The categorization of a large number of compounds as hallucinogens limits our ability to understand trends in harms with ICD-10 codes. Improved systems for surveillance of potential harms of hallucinogen use are needed. ### Competing Interest Statement The authors have declared no competing interest. ### Funding Statement This study did not receive any funding ### Author Declarations I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained. Yes The details of the IRB/oversight body that provided approval or exemption for the research described are given below: All data was publicly available from the California Department of Healthcare Access and Information. All original datasets that were used can be found on their website at and specific datasets that were used include and I confirm that all necessary patient/participant consent has been obtained and the appropriate institutional forms have been archived, and that any patient/participant/sample identifiers included were not known to anyone (e.g., hospital staff, patients or participants themselves) outside the research group so cannot be used to identify individuals. Yes I understand that all clinical trials and any other prospective interventional studies must be registered with an ICMJE-approved registry, such as ClinicalTrials.gov. I confirm that any such study reported in the manuscript has been registered and the trial registration ID is provided (note: if posting a prospective study registered retrospectively, please provide a statement in the trial ID field explaining why the study was not registered in advance). Yes I have followed all appropriate research reporting guidelines, such as any relevant EQUATOR Network research reporting checklist(s) and other pertinent material, if applicable. Yes All data produced are available online at