There's a sad but wonderful Snupin-deathfic I remember and muchly want to re-read, in which Remus takes a new potion to prevent his monthly changes. Although he's now free to wander under the fullmoon as a human being, Severus does not want him to continue taking the potion for there aren't yet any longtime experiences to prove its security for the werewolves' health. As it is, some years later Remus does become terminally ill by the continued intake of the potion. In the end, the tiniest move he makes causes him unimaginable pain. I recall the main story being framed by the final scene.
Does that ring a bell? It would be great, if someone had an idea where this fic is to be found.
Much love! Night
EDIT: Found! Many thanks to red_day_dawning!! I'm so happy now!