**Background:** Viral infections remain a major global health challenge, with licensed antiviral drugs often limited by adverse reactions and the rapid development of viral resistance. Natural products from plants offer a diverse source of specialized metabolites that can act through multiple mechanisms, potentially overcoming these limitations. This systematic review aimed to summarize the in vivo antiviral activity of plant-derived specialized metabolites and their mechanisms of action.
**Methods:** A systematic literature search was conducted in April–May 2022 across PubMed, Scopus, and SciFinder using a combination of keywords for antiviral activity, specific compound classes (e.g., flavonoids, terpenoids, alkaloids), and in vivo models. The review followed PRISMA guidelines. Two investigators screened titles, abstracts, and full texts; a third resolved disagreements. Inclusion criteria: in vivo studies (clinical or preclinical) testing single, natural (not semisynthetic/synthetic) compounds against viral infections. Exclusion criteria: extracts/fractions, reviews, meta-analyses, case reports, conference proceedings, non-English articles, and unavailable full texts. Data extracted included compound, source, virus type, experimental model, dose, mechanism of action, and outcome. Methodological quality was assessed using a standard checklist for randomization, blinding, and outcome measurement.
**Key Results:** The initial search identified 15,786 reports; after deduplication, 8785 articles were screened. Following title/abstract screening, 209 articles were fully analyzed, and 186 were included after adding 38 reports from manual searching. The first included study was from 1968, but 76% (141 papers) were published from 2010 onward. China led research with 95 documents (41.1%), followed by the USA (29, 12.6%), Japan (16, 6.9%), South Korea, and Taiwan. Quality assessment showed that 29.6% of studies reported randomized allocation, and 70.4% reported sample size calculations. The review categorized compounds into terpenes (monoterpenes, iridoids, sesquiterpenes, diterpenes, sesterterpenes, triterpenes, saponins, carotenoids, limonoids, steroids), flavonoids (flavones, flavonols, flavanones, isoflavones, chalcones, catechins), phenylpropanoids, lignans (classical, neolignans, flavolignans), tannins, alkaloids, diarylheptanoids, benzoic acids, coumarins, and other compounds. Key examples: 1,8-cineole (30-120 mg/kg) downregulated IL-1β, IL-6, TNF-α, IFN-γ in lung tissue of IAV-infected mice. Andrographolide (100-200 mg/kg/day) increased survival rate and decreased lung pathology in H5N1, H9N2, and H1N1 models. Baicalin (50-200 mg/kg/day) reduced viral load and lung inflammation in RSV-infected mice. Quercetin (240-960 mg/kg/day) improved survival rate (80% at 240 mg/kg) in IAV H1N1-infected mice. Silibinin (20 mg/kg/day intravenous) reduced HCV RNA levels in clinical trials. Oxymatrine (200 mg/kg/day) inhibited HBV DNA replication and reduced HBsAg/HBeAg in mice and patients.
**Clinical Implications:** The review highlights that many plant-derived compounds act through multiple mechanisms, including direct inhibition of viral entry/replication and modulation of host inflammatory and immune responses. This multi-target activity may reduce the risk of resistance development compared to single-target synthetic drugs. However, only a few compounds (e.g., silibinin for HCV, oxymatrine for HBV) have progressed to clinical trials, primarily due to challenges in patentability, bioavailability, and toxicity. The authors emphasize the need for rigorous preclinical and clinical evaluation, including assessment of PAINS (pan-assay interference compounds) and bioactivation toxicity. Despite these hurdles, natural product scaffolds remain valuable starting points for medicinal chemistry optimization to develop new antiviral therapies.