found this when looking up possible side effects of tysabri, and thought it was as good a description of fatigue as i’ve found:
Fatigue: A condition characterized by a lessened capacity for work and reduced efficiency of accomplishment, usually accompanied by a feeling of weariness and tiredness.
it’s been a long, weary week.
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one life: livin' the dream 
“feeling of weariness and tiredness”
Profound and overwhelming tiredness … a kind of tiredness that eclipses the consideration of anything else. Yess … that better.
The definition on all of the medical websites continues with:
::Fatigue can be acute and come on suddenly or ::chronic and persist.
This definition doesn’t begin to describe the medical-induced fatigue that many of us experience. There is no way to push past real fatigue; it simply consumes your mind and body past the ability to manage.
When you have to take a nap at 1:00pm after a nine-hour night of sleep, you know that this is bigger than your “will.”
Thank goodness my doctors don’t pooh-pooh fatigue. It is a known side effect of these drugs, and in combination, they throw some of us over the edge. If someone told me this was in my head, I might be on trial for assault.
Sometimes, with drug-induced fatigue, the fatigue lessens over time. Let’s hope that this is the case for you.
Be well.
*susan*
i am so sorry that you are having fatigue. how does one know if it caused by the MS or the medication?