Background: The effectiveness of ARV treatment among IDUs is still low. Objectives: Describe and compare the effectiveness of ARV among IDUs and non-IDUs at an outpatient clinic in Tu Liem, Hanoi. Methods: Qualitative design, secondary data were collected and 33 in-depth interviews with IDUs and 17 others health staff, family members were conducted. Results: The effectiveness of ARV treatment among IDUs is significantly lower than that of non-IDUs. The rate of weight gain, CD4 increase and opportunistic infection is lower among non-IDUs compared to IDUs: 52,6 percent vs. 63,4 percent
67,9 percent vs. 85,1 and 19,62 percent vs. 5,31 percent respectively. There is statistically significant association between being an IDU and CD4 count, weight gain and opportunistic infection. Conclusions: counselling for treatment for IDUs and their families on adherence need to be strengthened. Comprehensive treatment that integrates with ARV treatment should be provided for IDUs.