Is skin darkening that was caused by the sun irreversible? if not, how can it be reversed?

Dematology: some changes made by the sun are reversible, others are not and can be and early stage of cancer, or even cancer itself. when in doubt have a doctor take a look.

Reversible: it's usually reversible. Either you just let time readjust skin pigmentation(though you need to stay out of the sun too) or in some cases creams or other procedures including various laser / light therapies are necessary for non-"tanning" related hyperpigmentation. Some people using over the counter bleaching creams or citrus fruit juice, but this is more likely to irritate your skin (stay clear).

Tan: Skin darkening caused only by the sun is a tan and fades naturally.Some meds such as hormones and some diseases can also cause skin darkening which is harder to reverse.Lupus and pregnancy can cause facial darkening and adult onset congenial adrenal hyperplasia among other things can cause darkening of the neck and inner elbows.If you are having problems see your doctor to be sure you do not have