I think everything is interconnected in our health. Dr. Charles Parker talks about how allergies can affect ADHD medication efficacy here:
And there has been discussion for quite some time about how symptoms of allergies can look like ADHD symptoms.
Kenneth Bock links them together in his book, Healing the New Childhood Epidemics: Autism, ADHD, Asthma, and Allergies: The Groundbreaking Program for the 4-A Disorders.
ADDitude Community Moderator, Author & Mentor on Parenting ADHD, Mom to teen w/ ADHD, LDs, and autism