While kojic acid has been shown to lighten age spots, it can also cause an allergic reaction, impair thyroid function and cause unintended skin color changes, according to Skin Care Guide. Kojic acid is a naturally occurring substance derived from a fungus. It is effective in the short term to even out skin tone, clear acne and treat melasma. However, its long-term use is controversial, as it can leave lasting effects on the body.
An allergic reaction to kojic acid results in allergic contact dermatitis, according to Skin Care Guide. This condition causes discomfort. It happens in some people after long-term use and can be permanent. Kojic acid use has been linked to changing the skin's color abnormally, including creating a bluish tint when used over a long period. Studies have also discovered a link between kojic acid use and decreased thyroid function and other internal changes in the body. While long-term use can be problematic, short-term use has been very successful in alleviating acne in young people without the use of harsh chemicals and evening out skin tone in cases of hyperpigmentation. Anyone wishing to use kojic acid should first speak with a dermatologist to determine if it is an appropriate choice.