1 Medical Hypothesis: Caffeic Acid Domina Petric, MD Clinical pharmacology and toxicology University Hospital Center Split, Croatia ABSTRACT Caffeic acid, which is pharmacologically active compound of many traditional medicinal herbs such as Echinacea purpurea and Sambucus Formosana Nakai, might be a potent molecule for the treatment of COVID-19, especially if pared with Fe 3+ ions (caffeic acid Fe 3+ chelate) or other ions (MoO4 2- and PO4 3- chelates), because antiviral activity of caffeic acid increases upwards of 100-fold by the addition of cations (Fe 3+ ) and anionic molecules (molybdate, phosphate). It has also been reported that caffeic acid, which exhibits antioxidant, anti-inflammatory, antimicrobial and antiproliferative properties, has antioxidant property in normal cells and pro-oxidant property in cancer cells, what leads to pro-oxidant-mediated oxidative DNA damage and its downstream signaling induces apoptotic cancer cell death. Theoretically, caffeic acid and its derivatives have characteristics of a perfect chemotherapeutic agent that is protective for normal cells and destructive for cancer cells. Keywords: caffeic acid; COVID-19; cancer INTRODUCTION Echinacea purpurea (Asteraceae) is a perennial medicinal herb with important immuno-stimulatory and anti-inflammatory properties, especially the alleviation of cold symptoms. Different classes of secondary metabolites of the plant such as alkamides, caffeic acid derivatives, polysaccharides, and glycoproteins are believed to be biologically and pharmacologically active 1 . Echinacea purpurea has both antibacterial (against Streptococcus pyogenes, Haemophilus influenzae, Legionella pneumophila…) and antiviral properties (mostly against respiratory viral infections, including coronaviruses) and it is suitable for relieving symptoms of respiratory tract infections 2-10 . Echinacea purpurea medicinal preparations can be used for prevention, in the treatment of early phase and in mild respiratory infections. It has been reported in an in vitro study that Echinacea purpurea had antiviral activity against SARS- CoV and MERS coronaviruses 11 . Sambucus nigra products (Sambucol) have antiviral properties against different strains of influenza virus. Sambucol Elderberry Extract and its formulations activate the healthy immune system by increasing inflammatory cytokine production. Standardized elderberry liquid extract possesses antimicrobial activity against both Gram-positive bacteria of Streptococcus pyogenes and group C and G