Long-time listeners of Rush may recall that once a year or so, Limbaugh is absent for a day in order to attend to a required medical exam for certification of his 'Key Personnel' business insurance (or whatever they call it). This is the kinda stuff that CEOs have to do and others who are "THE" reason for a business's success.
Certainly, Rush would have blood work done that would reveal a drug problem - if it existed - and certainly the insurance rates would sky-rocket if that were true... if he could get business insurance at all with a drug problem. In turn, the corporate types surrounding his enterprise would step in Real Quick to take care of such a problem immediately.
So my thinking is that there is nothing to this other than typical prescribed medication for anti-rejection drugs or occasional pain.