They can lose potency. It depends on how they are stored.

I just bought a large stash of over 100 flavours second hand so have just been though this, some are still absolutely fine, some not so (the 70ml or so of CAP VC is completely stale and awful, which is an utter tragedy !!).

Some of the ones labelled 2018 are good, some that look a lot newer aren't, there doesn't seem to be any pattern of how much air in the container, what they are stored in etc. - even some of the fruits are great, I had assumed the strawberry and blueberry would be awful, but not so (the agreed price reflected the assumption some would be unusable).

The smell test is a good start, does it smell good - if they are flavours you know you want to keep using and will certainly buy more at some point then you could buy a fresh 10ml of those right away and you'll have a fresh one to compare with, then decide whether to ditch the old, keep them separate or as I've done with a couple, mix the old and new for a compromise.

You could also make up a flavour tester with the suggested solo mix %age for each and find out for sure.

If they're discoloured then yep, maybe not good any more but still worth checking, they might still be OK if you up the %age a bit, or a lot, I have a few that I am mixing at double the %age in my recipe and while they're not great, they're fine and cash is tight so I prefer not to just ditch them unless they are beyond help.

The CAP VC though, that'll go down the drain at some point, but I can't quite bring myself to do it yet !!

I just tossed about 15 bottles I've had for many years.. had turned brown, stored badly.. some of the first flavors Id bought yrs ago. Just small bottles of TFA. Obviously wasn't using them anyway

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