Amox & Allopurinol. There is a higher likelihood of developing a rash if you take Amoxicillin and Allopurinol together. It does not appear to occur often enough nor is it serious enough to warrant a change. If you are on Allopurinol and have to be on Amoxicillin for an infection, you can be monitored for a rash. If the rash occurs and is severe, a different antibiotic can be prescribed. The rash may not occur at all.