I just googled Metacam side effects in GPs - there's alot about cats and dogs .
One page about rats is very anti ( And I know rats ain't guineas and guineas ain't rats)
Side effects are liver and kidney problems- so UTIs, peeing problems, metabolism impaired if the liver isn't functioning.
And one page warned to only use for 3 days.
It's a can of worms!
What about homeopathic treatment for your GP? (I used Omega Oil and Evening Primrose Oil for my moggies eczema when prednisilone didn't help.It was out of desperation).
If one of my guineas was 7 (and I've had an elderly pig years ago, IIRC 8yo) I'd have to really sit and weigh up the pros & cons
Quality of life she's got now?Is she in pain?Is she enjoying life?
Will the medication help her enough to over-ride the side effects? Don't forget getting the medication into the GP!
Anything else (like low dose painkillers or herbal) to ease her pain?
Or let nature take it's course and when it's time, say goodbye.
I'm assuming you've got 2 old(ish) pigs here, what happens to pig 2 after pig 1?
No experience, no real advice.
Google and read all you can Forewarned is fore armed.
Is your vet a cavy savvy or mainly cat'n'dog?
Have you contacted Cambridge Cavy?
Or Google Peter Gurney. I read some of his pages, he uses alot of different things to treat GP ailments.
When did guinea-pigs get so complicated?
Good Luck.