My healthy dog keeps eating my arthritic dog's joint supplements — is that safe?

The short answer is: glucosamine and chondroitin are not toxic to dogs, so the odd stolen tablet isn't dangerous. But if this is happening regularly, the arthritic dog is being underdosed and the healthy dog is getting a supplement they don't necessarily need. How is the supplement being given, and which one is it?

That explains why you haven't seen improvement — the one who needs it has barely been getting any. The practical fix is to feed the older dog in a completely separate room or with the door between them during the supplement window, then reunite them. Alternatively, switch to the YuMOVE Senior liquid format if that's available in your region — it can be stirred into the food so it's harder to steal during bowl-swapping. The healthy younger dog taking the occasional tablet is fine, but consistent full-dose theft means the management plan has been failing without you knowing it.