Asking about drugs is a sure way of raising suspicions, so I am going to watch the thread closely, but the question is perfectly valid.
There is no single, simple answer.
In general every drug is just a chemical compound, in most cases an organic one. Most organic compounds will slowly decompose/oxidize in the presence of air, especially at elevated temperatures and in the presence of light. The larger and more complicated the molecule (and most drugs are far from being simple) the easier it decomposes (not a hard rule, but good enough for practical purposes).
Problem is: there are no more rules. Some compounds are quite stable and if properly stored (tightly closed dark bottle far from heat sources) have shelf life of many years. Some are quite fragile and even in low temperatures will decompose in weeks or even days (or even minutes, but these are rather unlikely to have any therapeutical value).
So the best answer that can be given is: question is valid, but it all depends on the exact compound/drug.