Background: There is a strong link between conflict exposure and ill health, including substance use. However, this widely acknowledged problem has not been studied yet in refugee camps in Lebanon.Aim: To investigate substance use among civilians following war or displacement, and to assess its association with socio-demographic characteristics.Method: Cross-sectional observational study carried out in three Palestinian camps in Lebanon using the World Health Organization Alcohol, Smoking and Substance Involvement Screening Test (ASSIST). Participants were Palestinian adults born in Lebanon and Palestinian and Syrian adults recently displaced from Syria due to war. The percentage of people reporting substance use and the associations between lifetime and last three months substance use and demographic features were assessed using a multivariate logistic regression.Results: In general, lifetime substance use was higher among Palestinians born in Lebanon compared to Syrians and Palestinians displaced from Syria (OR 7.241, 95% CI [3.781-13.869], P <0.0001). Results from ASSIST score during last three months showed that moderate and high-risk use of cannabis and cocaine were higher among Palestinians born in Lebanon than Palestinians and Syrians displaced from Syria. The multivariate analysis showed that women had lower lifetime (OR 0.188, 95%CI [0.080-0.442], P <0.0001) and lower last three months substance use than men, whereas single people were more likely to use substances than married people (OR: 2.78, 95%CI [1.588-4.866], P <0.0001). Tobacco was significantly associated with higher risk of substance use.Conclusion: These findings suggest a higher rate of lifetime substance use among Palestinians born in Lebanon than in Palestinians and Syrians recently displaced from Syria. Substance use is influenced by different socio-demographic factors in the two groups of refugees. However, many factors other than socio-demographic characteristics and refugee status may influence substance use, particularly quality of life and health status that should be assessed in future studies.