Other common dark colorations on the skin are called keratosis and consist of locally overgrown layers of skin that are dark primarily because there is more tissue than normal. A few of these turn into skin cancers of a much less dangerous kind than melanoma. Darkened regions of the skin occur as a result of abnormal tanning when the skin is sensitive to sunlight. Several diseases and many drugs can cause photosensitivity . Some of the same drugs may also cause patches of discolored skin known as localized drug reactions and representing an allergy to that drug.
Sunlight darkens an abnormal chemical in the skin of patients with porphyria cutanea tarda.
Arsenic poisoning and Addison's disease are among these causes. Normal in pregnancy , this brownish discoloration of the face can also happen with birth control pills that contain estrogen. Overall darkening of the skin may be due to pigmented chemicals in the skin. Silver, gold, and iron each have a characteristic color when visible in the skin. Several drugs and body chemicals, like bilirubin, can end up as deposits in the skin and discolor it.