Objective: To determine the safety and efficacy of H.suaveolens extract for therapy of influenza in healthy adults.Methods: The study was randomized double blind placebo controlled study conducted in 15 community and general hospitals from May to August 2006. The study subjects were healthy adults who had influenza-like symptoms and positive preliminary diagnostic test for influenza A or B from respiratory secretions. They were randomized to receive H.suaveolens extract 500 mg 3 times daily or the placebo 3 times daily for 7 days. The subjects were evaluated for the severity of symptoms related to influenza, adverse effects of the medications and the presence of influenza viruses from respiratory secretions at entry, day 4 and day 7 after treatment.Results: There were 39 subjects in the placebo group and 46 in the H.suaveolens group. There was a significant improvement in symptoms of the patients in both groups on day 4 and day 7 when compared with that at entry. However, the average duration of fever of the patients in both groups was not significantly different (3.1 days in the placebo group vs. 3 days in the H.suaveolens group, p=0.749). The recovery rates of influenza A and influenza B viruses from respiratory secretions of the subjects on day 4 and day 7 after treatment in both groups were not significantly different. A trend of less positive culture for influenza A virus in the patients receiving H.suaveolens extract (32.5%) compared with those receiving a placebo (47.1%) was observed. The compliance to medications was satisfactory. No serious adverse effects due to study medications were observed.Conclusion: H.suaveolens extract 1.5 grams per day for 7 days is safe but it is not effective in relieving influenza-related symptoms in adults with influenza. The lack of efficacy of H.suaveolens extract might be due to an insufficient dosage of the extract.