Would an aspirin a day harm a 12 yr old collie X suffering from arthritis ?

I'm not a vet,but have had a dog that had arthritis.Aspirin can cause ulcers and bleeding problems.and other pain relievers can be toxic.But like a human your pet may benefit from glucosamine

Dorset, I strongly advise you to have a discussion with your vet. Mrs. Hammer is right. Aspirin once in a while is okay to use but not as a daily fix. All other pain relievers that we use are toxic to a dog's system. Glucosamine does help with arthritis. Rimadyl is a prescription arthritis medication that has shown some excellent results in treating dogs with this problem. There are other drugs like this that are on the market too. Your vet may suggest trying the glucosamine supplements sold in pet stores to see how your dog responds to them before trying a prescription medication. But, never give your dog anything without first running it by your vet.

I agree with the other two. I asked about using aspirin when Trink first started having problems, and my vet put her on Tramadol instead. She said she doesn't like to put dogs on aspirin because of the issues with bleeding, since dogs are generally active and bump into things, etc. which increases the chance of bleeding. (Technically, aspirin decreases the blood's ability to clot, so something that would just cause a bruise normally could instead continue to bleed into the area if they are on aspirin for a long time.)