It does help to know the TSA guidelines. I had flown many times with my mother in her own wheelchair, including to London, with never a problem, until the security at the Sarasota, FL airport decided to not allow the tool bag that went with the wheelchair (needed for adjustments, emergency repairs, etc.) There was nothing sharp in the bag, however, no amount of reasoning could persuade this guard or the guard's supervisor. Our luggage had already gone through. I had to leave my Mom in her chair, run back downstairs, and fortunately found a kind guy on the airline desk who somehow managed to get the tool kit in the luggage. Fortunately, the plane was delayed so we did not miss the flight. While we were waiting for the delayed plane, the director from security services came up and apologized, saying they had checked the guidelines and I was correct -- wheel chair tool kits that went with the chair were an explicitly allowed item. He said in the future, that if such a circumstance occured to ask the guard to check the relavant TSA guidelines for the item (in this case wheelchair tool kit) as it is the fastest way to resolve an interpretive dispute.
I would tend to carry a printout of the relevant TSA guidelines over a prescription as you occassionally run into newer security personnel who are overly scrupulous about their security interpretations.