I have an 11 year old rabbit that seems to be in discomfort from arthritis and age related issues. He lives with a younger rabbit, and to keep the old rabbit in the area that is appropriate for a geriatric bladder a we use an 8 inch (20 cm) board as a fence.
Our vet agreed that he may be in some pain, and suggested a dosage of 25mg per 2 kilograms up to twice a day. This is similar to the recommendations found else where 5-10 mg per pound of dog’s weight for 12 hours. I am NOT suggesting this correct dosage for your pet, talk your vet first.
He also offered a method which included dissolving an aspirin in water, and providing that as his only water source. Because we are medicating a single bunny in a two bunny group, that was not an option for us.
So we immediately set out to figure out how to get less than a baby aspirin into one bunny in a two bunny family group. FYI - Google does not show any viable results for purchasing aspirin in solution.