Why don't people actually try and answer the question instead of saying stupid things that don't even pertain to the question? 
I'd like to know the answer to the question too since I have a 1 week old carbon that I removed to add medications. So can this dried out 1 week old carbon still be used to remove medication??? Yes or No will do.
There isn't a yes or no answer PaPeRo - life is rarely like that!
There are hundreds of different ways to make activated carbon and each carbon will have different properties. Whether or not it can be re-used, once dried, will depend on:
i) how much material has already been absorbed - which will depend on exactly what was in your tank when you were using it, how much you fed the fish and loads of other variables, and
ii) whether drying out has affected the bonding of the chemicals to the carbon (which is more complicated than simple physical absorption).
The life of activated carbon in a fish tank has been estimated to be about six weeks - but I have also seen other figures quoted. The question with meds is - what capacity has your carbon for absorbing that particular med, what level is in the tank and how is that capacity likely to be affected by other chemicals already absorbed. It's very unlikely that anyone will ever have done any relevant tests for that. To be absolutely sure - when you need to remove a med - use fresh carbon.
If you don't need to remove a med - then there seems to be virtually no value in using carbon at all (at least for freshwater tanks).
And DON'T try re-activating carbon sponges in a domestic oven. Carbon itself will be fine in an oven - but the plastic sponges that are impregnated with carbon for aquarium use will definitely not be!!
Commercial carbon activating processes use temperatures up to 750 deg C and often also control the composition of the air. I don't think using a domestic oven would necessarily remove all the organic compounds absorbed.
