There isn't any such thing as "nitrous oxide cartridges" for medical use.
Legitimate medical users do not use "cartridges."
Nitrous itself is unscheduled and PA law doesn't specifically ban it to my knowledge. HOWEVER, any gas marked for medical use (even air) is considered a drug by federal and PA statutes and is restricted to prescription use.
In this case you appear to be in more trouble for claiming medical use!