Shahrekord University of Medical Sciences Epidemiology and Health System Journal 2980-7891 13 1 2026 06 29 Estimating the Prevalence of People Injecting Drugs at Ages 18-45 in Shiraz, Iran: The Use of a Network Scale-Up Approach 16 21 10.34172/ehsj.26367 EN Ahmad Maghsoudi https://orcid.org/0000-0002-8607-8436 Mohammad Reza Baneshi https://orcid.org/0000-0002-6405-8688 Jafar Hassanzadeh https://orcid.org/0000-0002-1055-1418 Journal Article 10.34172/ehsj.26367 2025 02 17 2026 01 07 Background and aims: Accurate estimation of populations at heightened risk for Human Immunodeficiency Virus (HIV) infection is essential for effective healthcare intervention planning and resource allocation. This study employed the network scale-up method to estimate the prevalence of People Who Inject Drugs (PWIDs) aged 18-45 in Shiraz, Iran. Methods: A purposive sampling approach was used to recruit 768 individuals aged 18-45, who were interviewed using a self-administered questionnaire. The average social network size was estimated using 16 reference groups with known population frequencies. Post-correction factors were applied to adjust for transmission and relative network-size errors. Data were analyzed using SPSS version 25. Results: The mean reported social network size was 203 individuals. The crude estimate of PWIDs in the target age group was 1,812, while the adjusted estimate was 4,857 (95% UI: 4,231-5,483). The prevalence of PWIDs was estimated at 561 (95% UI: 488-633) per 100,000 individuals, with the highest prevalence observed among those aged 35-39 years. Conclusion: The estimated number of PWIDs significantly exceeds the current number of individuals receiving support from harm-reduction services. To effectively combat the spread of HIV, enhancing access to prevention and treatment services for this population is crucial.