Always check with your vet before you add supplement to their diet.
A particularly important one to be careful one is turmeric. It can interact with other meds, NSAIDs in particular, can potentially cause stomach ulcers and bleeding issues. (Same for people, my mum wasn’t allowed it because she’s already on blood thinnners.). A good quality controlled turmeric supplement, specifically for dogs, given at the right dose shouldn’t cause problems, but golden paste or turmeric powder added by the spoon/half-spoon can vary a lot in terms of quantity and absorption. I do have my own elderly dog on turmeric supplements and it could be owner placebo, but we all felt that after a few weeks of being on it there was an improvement in how stiff he was and a willingness to move more.
One of my favourite supplements is high quality omega 3, from cold water fish. It’s one of the few that also has some decent research suggesting it’s beneficial. It has the added bonus of giving them a beautiful soft, shiny coat.
I also use Riaflex and Green Lipped Mussel for his Osteoarthritis. He’s now been on them a few months and just this week we have seen a big improvement . He’s more fluid in his movement and for the first time in a year, can squat to poo without having his legs tremble and needing to move and drop bits as he goes. He can now squat and poo in one spot and actually look comfortable.
Finally, he’s on Aktivait - on the advice of our vet, as he was starting to show signs of cognitive decline at the start of this year. I was very skeptical about this one, especially as it’s so expensive, but I have to admit it seems to have helped a lot. (Although dealing more effectively with his arthritis pain may also be a big factor in that.)
I’d add the caveat that, whilst we have added the supplements, he is also on NSAIDs from the vet and has been for a few years now. Also, we have been introducing a lot of other changes to his management over this year, based on advice from the Canine Arthritis Management’s website, so it’s more likely to be the combination of holistic changes, rather than any one element.
CAM have a list of common supplements, with advice and explanation of any scientific evidence for each one [https://caninearthritis.co.uk/managing-arthritis/diet-and-nutrition/available-supplements/ [here]].