In this study, the aqueous extract of bearberry leaves (Arctostaphylos uva-ursi L.), which is frequently self-administered in combination therapy with the antibiotic nitrofurantoin for the treatment of urinary infections, was subjected to a qualitative and quantitative analysis. The total polyphenol composition and antioxidant potential of the extract were determined spectrophotometrically, and the individual compounds arbutin and hydroquinone were determined by high-performance liquid chromatography (HPLC). The in vitro biological effect of the extract on the cell lines of the gastrointestinal tract (CAL 27, AGS, Caco-2) and liver (HepG2) were assessed by spectrophotometric techniques. In vitro digestion was used to determine the stability of the extract during intake. Using the Neutral Red, DCFH-DA, and micronucleus cytome assays, the bladder cancer cell line (T24) was utilised as a model system to examine the cytotoxic, prooxidative, and genotoxic effects of hydroquinone, nitrofurantoin, and their combination. Additionally, mass spectrometry was used to identify the differences in protein expression in T24 cells following treatment with the abovementioned substances. The major findings of this study demonstrate the synergistic effects of hydroquinone and the antibiotic nitrofurantoin on human urinary bladder cancer cells (T24), including decreased survival, increased pro-oxidative effect and genotoxic effect, and altered protein expression. It follows that herbal compounds should be viewed as chemical substances that, depending on how they are used, may exhibit synergistic, antagonistic, and potentiating effects when combined with other therapies, and may have a negative influence on human health.