Gluten can be hidden in many items you'd never suspect especially over the counter and prescription medications. Often prescription medications will use a wheat based binder and it won't be listed on your bottle or in the package insert. When dealing with generics this can vary by supplier ( ever notice your refill tablet or capsule looks different than what you expected?) and without notice.
Many times our customers complain of feeling glutened or off and don't know where the source is from. Our first question is usually have you checked your prescriptions or OTC items.
Here are two links that you may find helpful