Tylenol (acetaminophen) is a no-no for rabbits. And if the pain is caused by inflammation, you really want something that is anti-inflammatory as well. Aspirin dosage is tricky, but do-able-- one low dose aspirin (81 mg) will treat 40 pounds of rabbit,. So, grind up a tablet, split into 4 equal parts, and if the rabbit weighs 10 pounds, you have 2 days worth of pain/arthritis relief for it.[One dose every 12 hours]
The cambium layer of willow is what contains the salicylic acid (aspirin!!)that is so beneficial-- If willow is not available, this lovely little chemical is also found in the cambium layers of the aspen/poplar family trees.