Does bleach powder go off?

Does it have any best before date on it? Any recommendations on the pack?

It should be ok as long as there's no chance it's got damp or anything like that. I don't know how likely bleach powder is to degrade over time, but it's reasonable to assume it would so it might be worth doing a strand test to see if it's still effective.

I've used it after a year before, I think it can lose potency so I would definitely do a strand test but you may find it's fine! (a hairdresser told me that it keeps about 6 months once opened, but I've had it much longer than that in a sealed container before now.

I had some in an unsealed opened packet for over a year and it was hair disaster time. Since that happened I've been wary and will always buy new from now on.