Metacam (or drug name meloxicam) is a very safe drug. There's often a lot of hype which goes around about it being bad for guts and for kidneys and this is certainly the case when it is used for years and years long term on cats and dogs. Guinea pigs gastrointestinal systems cope very well with this drug and it doesn't cause gut ulceration, it also seems to be ok for kidneys. This drug is out first line when it comes to pain relief and exotic vets use it on a daily bases with good reason too. I wouldn't shy away from it personally and many piggies are much more comfortable on it. It's is a type of drug called a non steroid anti-inflammatory (NSAID), which is perfect for arthritis as it has the pain reliever effect as well as reducing inflammation in the joints. To add my dog has been in another type of NSAID (rimadyl) for the past three years despite having liver disease and he has not had an issues with his kidneys or gut ulceration. Had many a piggie on metacam too with no issues whatsoever, they love the stuff, have two oldies on it long term at the moment.
Tramadol is an opioid medication, it is stronger than metacam and I would personally not use as a first line, but in cases where your worried about kidneys or metacam on its own is not enough then tramadol is fine to add into the mix. There are some studies in dogs and humans which suggest this drug works in some patients and not in others, my dog for instance doesn't seem to get any pain relief from it, but I have had piggies who do. It can cause some degree of sedation in some animals depending on dosage and the individual.
Another type of medication useful for arthritis is gabapentin which is great for neurological conditions and nerve pain. It's worth considering along side metacam if you need it later on.
Hope this helps.
