IMPORTANCE: The lack of representative research on homelessness risks mischaracterizing and misrepresenting the prevalence of illicit substance use. OBJECTIVE: To estimate the population prevalence and patterns of illicit substance use, treatment, nonfatal overdose, and naloxone possession among people experiencing homelessness in 1 US state. DESIGN, SETTING, AND PARTICIPANTS: This representative survey study of adults experiencing homelessness from October 2021 to November 2022 in 8 California counties used multistaged probability-based sampling and respondent-driven sampling. Eligible individuals were 18 years or older and met the federal definition of homelessness. MAIN OUTCOMES AND MEASURES: The primary outcome measures included lifetime and past-6-month illicit substance use and substance type (methamphetamine, nonprescription opioids, or cocaine). Lifetime and current substance use treatment, unmet treatment need, types of treatments received, nonfatal overdose (lifetime and current episode of homelessness), and current possession of naloxone were measured. Population prevalence estimates with 95% Wald CIs were calculated using survey replicate weights. RESULTS: Of 3865 individuals approached, 3042 (79%) participated and an additional 158 participants were recruited through respondent-driven sampling. Among 3200 participants, the mean age was 46.1 (95% CI, 45.3-46.9) years, 67.3% (95% CI, 65.2%-69.3%) were cisgender male, and there were similar proportions of Black and African American, Hispanic and Latine, and White participants. Overall, an estimated 65.3% (95% CI, 62.2%-68.4%) of participants used illicit drugs regularly (≥3 times per week) in their lifetime
41.6% (95% CI, 39.4%-43.8%) began using regularly before their first episode of homelessness and 23.2% (95% CI, 20.5%-25.9%) began using regularly after. In the past 6 months, an estimated 37.1% (95% CI, 32.9%-41.3%) of participants reported regular use of any drug
33.1% (95% CI, 29.4%-36.7%) reported use of methamphetamines, 10.4% (95% CI, 7.9%-12.9%) reported use of opioids, and 3.2% (95% CI, 1.8%-4.6%) reported use of cocaine. In their lifetime, an estimated 25.6% (95% CI, 22.8%-28.3%) injected drugs and 11.8% (95% CI, 9.8%-13.8%) injected drugs in the past 6 months. Among those with any regular lifetime use, an estimated 6.7% (95% CI, 3.8%-9.5%) of participants were currently receiving treatment. Of those with any regular use in the last 6 months, an estimated 21.2% (95% CI, 17.9%-24.5%) reported currently wanting but not receiving treatment. An estimated 19.6% (95% CI, 17.4%-21.8%) of participants had a nonfatal overdose in their lifetime and 24.9% (95% CI, 21.3%-28.5%) currently possessed naloxone. CONCLUSION AND RELEVANCE: In a representative study of adults experiencing homelessness in California, there was a high proportion of current drug use, history of overdose, and unmet need for treatment. Improving access to treatment tailored to the needs of people experiencing homelessness could improve outcomes.