If you want something vaguely plausible in a scifi setting then you might go with a chrysalis. Every month would be far too fast and the change happening in a week would require high-scifi stuff like nanotech for anything the size of a human but you could have some kind of vaguely biologically plausible cyclic transformation if you allow more time.
You could probably spin it as some kind of parasite which causes the transformations which eventually reverts you to human form to hide and as part of some kind of cyclic lifecycle which gives the advantage that it. This also gives you an excuse to make your werewolves immortal like Turritopsis dohrnii.
http://en.wikipedia.org/wiki/Turritopsis_dohrnii
The human ages and at some point feels a massive urge to eat. They fatten up then feel the urge to find somewhere secluded where they curl up and exude a chrysalis. A few months later your werewolf crawls out. Drama happens with wolf form and others are infected and eventually the wolf form feels the urge to find somewhere secluded again where it pulls the same trick to turn back into a youthful human who crawls out with vague memories of doing something terrible.