While the Brighten & Tone Vitamin C Serum is generally well-tolerated, some users may experience side effects such as mild skin irritation, dryness, stinging or tingling sensation, increased sensitivity to sunlight, breakouts, or temporary yellowing of the skin. To minimize these effects, start with a lower concentration, perform a patch test, and always use sunscreen during the day. If any severe irritation or adverse reactions occur, discontinue use.