INTRODUCTION:
This systematic review examines the prevalence of substance use and related disorders (SUDs) across South Pacific nations, addressing limited and unevenly reported epidemiological evidence and highlighting implications for public health policy and equity.

METHODS:
Evidence published from 2000 onwards was reviewed for South Pacific countries including Samoa, Papua New Guinea, Fiji, and French Polynesia, while excluding Hawaii, New Zealand, Australia, and American Samoa. Prevalence studies indexed in Medline, CINAHL, PsycInfo, Embase, and the Cochrane databases were screened and retrieved. Included studies were critically appraised using Joanna Briggs Institute (JBI) tools.

RESULTS:
Ten studies were included, employing varied designs (including national surveys). Prevalence estimates differed widely across countries and by sex and ethnicity. Tobacco and betel nut use emerged as particularly common across settings. Indigenous populations and men consistently experienced a higher burden of substance use and SUD-related harms.

CONCLUSIONS:
Substantial methodological inconsistencies, especially differences in measures and reliance on self-report, limit direct comparisons across countries and populations. Evidence on co-occurring mental health disorders and polysubstance use remains scarce, indicating major research gaps. The review underscores the need for culturally sensitive, standardised epidemiological research and improved surveillance systems to track emerging trends (including synthetic drug use). Policy responses should prioritise targeted prevention and treatment interventions that reduce disparities and strengthen support for vulnerable Pacific Island communities.