Polyphenol and Flavonoid Content and Antioxidant Activity of Hypericum perforatum L. (St. John’s Wort) Extracts for Potential Pharmaceutical and Cosmetic Applications

St. John’s wort (Hypericum perforatum) has been used for centuries in traditional medicine owing to its high content of various bioactive metabolites and wide geographic occurrence. Nowadays, it plays an important role in the pharmaceutical industry and is increasingly significant in modern cosmetology. The objective of this study was to assess the antioxidant activity and compare the content of polyphenolic compounds in two commercial extracts of H. perforatum, glycerol–water and propylene glycol–water, which are used as cosmetic raw materials. The HPLC method was used to determine phenolic compounds. The total polyphenol content and total flavonoid content of H. perforatum extracts were determined using spectrophotometric methods. Free radical-scavenging properties were analyzed using a 2,2-diphenyl-1-picrylhydrazyl radical assay with electron paramagnetic resonance spectroscopy (DPPH-EPR assay), as well as the ferric reducing antioxidant power (FRAP) method. St. John’s wort extracts were able to scavenge free radicals, indicating beneficial cellular protection against oxidative stress. The use of non-toxic extractants makes it possible to obtain extracts with high antioxidant potential, which can be safely used in the pharmaceutical and cosmetics industries. The results of this study, i.e., the values for TPC, TFC, and antioxidant activity (DPPH and FRAP), suggest that Hypericum perforatum, especially the glycerol–water extract, has antioxidant potential.