Could 2% hydroquinone cause irreversible darkening long term or is this only with higher concentrations? (this really gave me doubts) advice? thanks

Typically not: The recommendation is to use hydroquinone for a few months then take a break. But otherwise I think its fine

Complicated: Long- term, uninterrupted use at any strength has potential for paradoxical darkening of skin. This is VERY difficult to reverse. Best use is to stop using it after a few months and not resume for several weeks.

Skin pigmentation : Hydroquinone is used to reduce pigmentation of skin. OTC preparations are 2% and stronger preps are 4% and as high as 10%. An unusual reaction of this medicine can be bluish dark discoloration of skin called Onchronosis Cause se is not known but it us rare.