I think your doctor would know the most. The general info I have--from being a patient--is that the drug leaves your system in a matter of days but that side effects can persist for a month or more. But it is worth asking your oncologist directly. Fatigue can also come from much of cancer treatment. How long ago was the radiation? I thought my fatigue was just emotional but turns out my white count was down post radiation. Can you get clearer direction from the oncologist so you have more of a timeline? I've found it hard at times to understand what was causing specific symptoms. I hope you can get more information.