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Background: Long COVID have posed a global health burden since the COVID-19 pandemic. This study aimed to evaluate the efficacy and safety of a combined plant extract (CPE) formulation, containing Citrus aurantifolia, Tiliacora triandra, Cannabis sativa, Alpinia galanga, and Piper nigrum, in participants with long COVID. A newly developed long COVID symptom questionnaire was used to evaluate outcomes. Methods: This randomized, double-blinded, placebo-controlled trial was conducted at the College of Pharmacy, Rangsit University, Thailand. Participants were randomly assigned to receive either a CPE supplement (4500 mg/day) or a placebo for 7 days. Primary outcomes were changes in C-reactive protein (CRP) levels and the total symptom score (ranging from 0 to 57 points). Secondary outcomes included full recovery/improvement of long COVID symptoms, health-related quality of life (HRQOL), and adverse events. Results: A total of 66 participants were enrolled, with 33 in each group. The CPE supplement did not significantly reduce CRP levels, with a median difference (MD) (95 % CI) of −0.05 (−0.49, 0.39) mg/L compared to placebo. However, the CPE group showed a reduction in the total symptom score [MD (95 % CI) of −4.00 (−7.58, −0.42)], and a reduction in overall moderate to severe symptoms [RR (95 % CI) of 0.57 (0.35, 0.91)], moderate to severe fatigue [RR (95 % CI) of 0.25 (0.08, 0.81)], and moderate to severe post-exertional malaise (PEM) [RR (95 % CI) of 0.35 (0.16, 0.78)]. Changes in HRQOL scores did not differ significantly between groups. Adverse events were mostly mild and resolved by the end of the follow-up period. Conclusions: Our study suggests potential benefits of the CPE in alleviating moderate to severe long COVID symptoms, particularly fatigue and PEM, with an acceptable safety profile. However, larger-scale trials are necessary to validate these findings, and assessing the reliability of the long COVID symptom questionnaire is essential before its application in future studies. Trial registration number:: TCTR20230131004 (Registration date: 2023–01–31, Thai Clinical Trials Registry). more...