Anethole, Antimicrobial Properties, and Digestive Health

Evidence Review

De et al. (2002) — Foundational antimicrobial study [1]

This early laboratory investigation in Phytotherapy Research established the antimicrobial basis for star anise's traditional medicinal use. The authors demonstrated that star anise extracts and isolated anethole were effective against a range of bacterial and yeast species. Critically, they showed that the bulk of the antimicrobial activity could be attributed to anethole — providing the mechanism-of-action foundation that subsequent research has built on. The study tested multiple extraction solvents and found that both essential oil fractions and ethanolic extracts were active. While this is in vitro work, it established the dose-response relationships and species-level antimicrobial activity that informed later research. Limitations include the absence of human pharmacokinetic data and the gap between in vitro MICs and achievable tissue concentrations from dietary consumption.

Sharafan et al. (2022) — Comprehensive review of medicinal and cosmetic applications [2]

This review in Molecules is the most comprehensive recent synthesis of Illicium verum and trans-anethole research. The authors catalogued: pharmacological activities (antibacterial, antifungal, anti-inflammatory, antioxidant, anxiolytic, antinociceptive, gastroprotective, expectorant, estrogenic); trans-anethole content in the essential oil (72–92%, with the remaining fraction comprising foeniculin, anisaldehyde, and methylchavicol); current safety consensus; and cosmetic applications. The review confirmed the safety profile of trans-anethole at food and supplement-relevant doses and noted that most adverse events in the literature involve accidental ingestion of Japanese star anise (Illicium anisatum) rather than culinary star anise. Of particular note is the discussion of trans-anethole's estrogenic activity — while weak, it may contribute to the traditional use of star anise for menopausal symptoms and lactation support, though clinical human evidence for these applications remains limited.

Huang et al. (2010) — Antifungal activity of essential oil and trans-anethole [3]

This study in Molecules tested the essential oil of Illicium verum fruit — characterized by GC-MS as 89.5% trans-anethole — against eleven species of plant pathogenic fungi including Alternaria solani, Bipolaris maydis, Fusarium oxysporum, and Sclerotinia sclerotiorum. Both the whole essential oil and isolated trans-anethole showed dose-dependent antifungal activity in mycelial growth inhibition assays. Trans-anethole alone accounted for the majority of the antifungal effect, confirming it as the principal bioactive compound rather than a minor constituent. The study also characterized the secondary compounds (cis-anethole at 0.7%, foeniculin, anisaldehyde) and found their individual contributions were minor. While the focus was plant pathogenic fungi, the results have been cited extensively in the context of Candida and Aspergillus research. The in vitro conditions do not replicate gastrointestinal or systemic exposure in humans; bioavailability and achievable local concentrations in the gut lumen from dietary or supplemental star anise remain areas for further study.

Patra et al. (2020) — Chemical compounds, antiviral properties, and clinical relevance [4]

This detailed review in Phytotherapy Research (34 pages, 150+ references) addressed the full phytochemical composition of star anise and systematically reviewed its antiviral evidence. The authors confirmed that shikimic acid — present at approximately 6–8% of dry weight in star anise fruit — is the precursor to the neuraminidase inhibitor oseltamivir (Tamiflu), making Illicium verum the world's primary commercial source of this pharmaceutical starting material. Importantly, the review also examined direct antiviral activity of star anise extracts: multiple laboratory studies showed inhibition of influenza A and B strains, with mechanisms including neuraminidase inhibition and interference with viral attachment to host cells. Additional antiviral effects against herpes simplex, hepatitis B, and Newcastle disease virus were also documented in cell-based assays. The authors were appropriately cautious about translating laboratory findings to clinical recommendations, noting the absence of adequately powered human trials for antiviral endpoints. The review also catalogued the phenylpropanoids, flavonoids, lignans, terpenoids, and sesquiterpenes present in different plant parts, providing a useful map of the pharmacologically active fraction.

Iftikhar et al. (2022) — Anti-obesity and antioxidant effects of star anise tea [5]

This preclinical study in Antioxidants used a high-fat-sugar diet (HFSD) rat model to investigate star anise tea at two doses (250 and 500 mg/kg body weight daily). Compared to untreated obese controls, both doses produced significant reductions in body weight gain, total cholesterol, LDL cholesterol, triglycerides, and markers of oxidative stress (including TBARS and DPPH radical scavenging measurements). The higher dose (500 mg/kg) also improved HDL cholesterol. The authors identified the dominant polyphenols in the tea preparation as p-coumaric acid, gallic acid, cinnamic acid, chlorogenic acid, and ferulic acid, with catechin and rutin as the principal flavonoids — a distinct polyphenol profile from that produced by essential oil extraction, highlighting that aqueous tea preparations deliver different bioactive fractions than anethole-rich oils. The rat model results are consistent with the anti-inflammatory and lipid-lowering mechanisms established in other star anise research, though dose translation to humans is inexact. Human trials using aqueous star anise preparations for metabolic endpoints have not yet been published.

Mosaffa-Jahromi et al. (2024) — Randomized controlled trial in COVID-19 patients [6]

This is the strongest clinical evidence for star anise's digestive effects. Conducted at Shiraz University of Medical Sciences (Iran), the study enrolled 225 non-hospitalized adults with confirmed acute COVID-19 infection who reported at least one gastrointestinal symptom within the preceding 48 hours. Participants were randomized 1:4 to receive either 25 g of powdered aniseed (Pimpinella anisum, a related anethole-rich species) twice daily or placebo for two weeks, added to standard care. The aniseed preparation contained 87–94% trans-anethole as its primary active compound. The primary findings: participants in the aniseed group were significantly more likely to report improvement in abdominal pain (p<0.05), anorexia (p<0.05), and diarrhea (p<0.05) compared to placebo. Nausea, vomiting, and bloating did not show statistically significant differences. Adverse events were mild and similar between groups (two in the aniseed arm, three in placebo). Methodological strengths include double-blinding, placebo control, and standardized symptom assessment. Limitations include the 1:4 allocation ratio (which reduces statistical power in the treatment arm), the use of Pimpinella anisum rather than Illicium verum (though both are trans-anethole sources), and focus on a specific clinical population. The study nonetheless represents meaningful human clinical confirmation that trans-anethole-containing preparations improve gastrointestinal symptoms, supporting the traditional use of star anise and anise for digestive complaints.

Strength of evidence: The antimicrobial, antifungal, and anti-inflammatory effects of star anise and its primary compound trans-anethole are well-established in laboratory and preclinical models. One clinical RCT provides direct human evidence for digestive benefits. The antiviral effects, while mechanistically plausible, remain largely preclinical. The metabolic and anti-obesity data comes from animal models only. Overall, star anise has a solid evidence base for its traditional digestive and antimicrobial uses, with promising but less mature evidence for systemic metabolic effects. It is well-tolerated at culinary doses. The main safety caveat is ensuring the use of Chinese star anise (Illicium verum) rather than the toxic Japanese species (Illicium anisatum).