The issue of the effects of radiation on medication potency is not only important for long term stays in the ISS, but also very important for longer duration journeys and even for colonization.
One thing is certain, the chances are that astronauts/colonists will fall ill, not necessarily to the extent asked in How is the ISS equipped to deal with the outbreak of a contagion?, but with the more mundane illnesses mentioned in the answers. Some would require regular medication as well.
So, the question is, in the increased radiation environment of space, how does increased environmental radiation affect medication potency, in the long and short term?
Additionally, are medications on the ISS shielded to protect it from radiation?