Hyperpigmentation or Dark Sports or Melasma.
Name the Dark skin Discoloration, Hydroquinine and Kojic Acid are extensively used and abused.
Hydroquinone for sure is a a gold standard active that almost every dermatologist will vouch for in terms of fading the pigmentation. But, hydroquinone bleaches your skin ( decreasing the number of melanocytes (cells make melanin, which is what produces your skin tone) present) which is quite useful in treating a variety of pigmentation disorders. That being said, the de-pigmentation activity of hydroquinone comes from a concept of "Cytotoxic" whitening which means, whitening activity through cell death. Sometimes, hydroquinone has caused a condition called Ochronosis (Sometimes irreversible bluish-black pigmentation on the face) through a prolonged use of the product. Also, Hydroquinone has tendencies to vigorously irritate and sensitise the skin which is why most of the products you find containing Hydroquinone are coupled with Tretinoin (Vitamin A) and a steroid like Mometasone or Fluocinolone (Medium strength corticosteroid typically used to treat various skin conditions like Eczema, Dermatitis) which on prolonged usage can enter into the bloodstream and cause potential problems and disrupts the stratum corneum " Epidermis of the skin".
Kojic Acid One of the First Generation De-Pigmentation active as an alternative to Hydroquinone. Acclaimed as one of the best next to hydroquinone as it does not need to be coupled with corticosteroids. Kojic Acid is derived from various fermentation processes usually with malting rice. Kojic Acid is a potent tyrosinase inhibitor works good when combined with a good exfoliating active such as Glycolic Acid. Kojic Acid containing products are in the top 10 choices of many dermatologists to treat various pigmentations. On the Contrary, Kojic Acid also has a bleaching effect on the skin through cell death whitening. Side effects of Kojic Acid which are frequently observed are sensitisation of the skin, Contact Dermatitis, and irritation.
Summarising the reasons avoiding Hydroquinone & Kojic Acid,
Irritation
Contact Dermatitis
Sensitisation of Skin
Cytotoxic Skin Whitening ( Whitening action through destruction of Cells)
Ochronosis