Some side effects of malic acid when taken as a supplement include nausea and diarrhea, according to About.com. When malic acid is applied to the skin, side effects include redness, itching and irritation.
Malic acid is a natural substance found in apples, explains About.com. It is considered an alpha-hydroxy acid, the type of acid commonly associated with skin-care products. Malic acid is also sold as a dietary supplement, having a variety of benefits. It is sometimes used to improve an athlete's performance, when taken as a supplement. Many claim that this product can fight off fatigue and boost a person's endurance during exercise.
Malic acid, when consumed together with magnesium, potentially relieves symptoms of fibromyalgia, including pain and chronic fatigue syndrome, notes About.com. However, as of 2015, research doesn't necessarily support that claim, and more studies need to be conducted to further determine its validity. Malic acid and magnesium can both cause intestinal issues, so those consuming both should stop taking the supplements to determine if the symptoms disappear.
When malic acid is applied to the skin, it helps reduce the signs of aging, including wrinkles, states About.com. It also reportedly helps in the treatment of acne, removes dead skin cells, and helps enhance the production of collagen, a protein that is a major component of skin.