Posttraumatic stress disorders often co-occur with depression and/or anxiety, leading to high levels of distress and low quality of life. Up to 70% of people with posttraumatic stress disorder do not achieve symptom remission from first-line pharmacological and psychotherapeutic interventions. Micronutrient interventions have demonstrated efficacy in treating acute posttraumatic stress and improving resilience after a disaster. This study investigated the efficacy of a micronutrient intervention for improving posttraumatic stress symptoms in people with functionally impairing anxiety and/or depression recruited to a 10-week randomised controlled trial (RCT) with 10-week open label (OL) phase. Seventy-two participants of 117 recruited endorsed a previous traumatic event and were included in the present study. There were no significant group differences on Impact of Event Scale – Revised between the micronutrient and placebo conditions at the end of RCT (between groups Effect Size (ES) d = 0.14, 95% CI [-0.53, 0.8]) nor at the end of OL (d = 0.27, 95% CIs [-0.35, 0.88]). Similar ESs were found for Depression, Anxiety and Stress Scale (DASS) total, depression and anxiety subscales, Patient Health Questionnaire–9 and Generalised Anxiety Disorder-7 Question Scale. The micronutrient group had larger ESs for DASS stress subscale at end of RCT and OL, and the end of OL Quality of Life Scale. Micronutrient expectation, treatment condition guess and past history of psychotropic medication were found to predict treatment response. This study found a 20-week micronutrient intervention was not superior in reducing chronic posttraumatic stress symptoms in people with co- occurring anxiety and depression over placebo. Future research may focus on predictors of treatment response including micronutrient expectation and past psychoactive medication use.