as in 5 years out of date - (found an old stash in the garage). What's the worst that could happen if I were to give it to the oldie - it won't work or will it kill her?!
I'm no expert, but at that age I'd bin it. As I understand it, some drugs, such as aspirin, change their chemical makeup as they age. Probably the worst is that it won't do any good, but bute is relatively cheap so I'd ask the vet for some fresh stuff.
The expiry date will be based on the manufacturers stability data. Up to the expiry date the stuff is statistically 'guaranteed' to remain within specification. Beyond that there'll be no data to support that it meets spec. This could mean that its absolutely fine or that it's lost potency and therefore won't be as effective. It could also degrade in a manner which produces undesirable side products and beyond the expiry date the levels could be above acceptable limits. I've no idea how bute degrades over time. I think it's highly unlikely it'd kill her but I'd bin it and get some in date stuff.