Let me muddle things up with some slightly informed guesswork. The structure/conformation of the active ingredient molecules of the medicine shouldn't be affected by freezing. As temperature drops, molecules "wiggle" less and so are less likely to accidently change into something else.

However, there's a lot more stuff in medicine than just the active ingredient. In dry pills there's time-release coatings, dispersion agents, fillers, binders, and assorted other things which can be affected by lower temperatures. My main concern would be with the time-release coating becoming more brittle and cracking when cold, thereby allowing the active ingredient to hit the body quicker and in a bigger dose than recommended.

In liquid medicines there is a different thing to worry about. Usually the medicinally active molecules do not want to disolve in water so they are dispersed/suspended in the water by surfactants. The ability of surfactants to suspend the medicinal molecules changes with temperature. If the temperature drops too low an irreversible phase-change can occur which destroys the ability of the molecules to stay suspended in the water.

I have no idea if the above concerns are true for medicine, but they are things I have to worry about when designing chemical blends for oil field applications, which is surprisingly similar to medicinal chemistry. "Winterized" chemical blends for the field are a pain in the @$$.

-Blast