Hydroquinone side effects serious?

Irritation. Used properly it should be limited to irritation. Overuse could rarely cause systemic effects.

Related Questions

Prescribed hydroquinone today any serious side effects I should watch out for?

Safe. Depending upon the concentration of hydroquinone and whether it is compounded with other medications including retinoic acid, it may produce localized inflammation. Otherwise, in lower concentrations, it is usually safe. Read more...