Objective: Drug addiction is a chronic disease. Its withdrawal causes unfavorable and averse symptoms such as anxiety. The objective of this study was to investigate the effect of Zataria multiflora on reducing anxiety caused by opioid withdrawal syndrome. Methods: The present study is a randomized and double-blind clinical trial conducted on 40 people with opioid withdrawal syndrome. They were selected from Isar Rehabilitation Clinic of Semnan City. They were randomly placed in two groups of Zataria multiflora essential oils (n = 21) and placebo (19). Beck Anxiety Inventory (BAI) was completed in the pre-test, post-test, and follow-up stages by subjects to measure their level of anxiety. Data were analyzed using SPSS 21 software. Results: The results revealed no significant difference between Zataria multiflora and placebo in the pre-test stages (p> 0.05). In the post-test and follow-up stages, Zataria multiflora could significantly reduce the anxiety of people with opioid withdrawal syndrome compared to the placebo (p <0.05, f = 5.064). The results of the LSD test also revealed a significant difference between the pre-test and post-test, and follow-up stages (P <0.05). However, no significant difference was observed between the post-test and follow-up stages (P> 0.05). Conclusion: The results revealed that Zataria multiflora can reduce the anxiety of people with opioid withdrawal syndrome. However, considering the limitations of the present study and a few studies conducted in this field, it is necessary to conduct similar randomized clinical trials.