* Chemical Reactions: The chemicals in hair dye (developer, colorant, etc.) can degrade and react with each other over time, even in a sealed container.
* Oxidation: While the bottle is sealed, some oxygen can still seep in over a long period, which can oxidize the ingredients.
* Separation: The different components of the dye can separate, making it difficult to mix properly and achieve the desired color.
* Expiration Dates: Most hair dye products have expiration dates. Check the packaging for this date. Using the dye after the expiration date can lead to unpredictable results, scalp irritation, or the dye simply not working.
Even if there is not expiration date. It is generally recommended to discard the hair dye after two years.