The sensation of slow withdrawal from the effects of a drug. Comedowns vary from drug to drug, but in general, with downers they're like a slow reassessment of reality, all the bad things and all the good are overwhelmingly intense. A bit like waking from a pleasant dream.
From uppers, the comedown leaves you numb, cold, lifeless and astonishingly fragile as well as physically sick.
To be avoided or slept through if at all possible.