They don't seem to fit neatly into the GTD system. Here's an example: each month I try out some changes in health, and one thing I'm doing this month is taking Omega 3 every day.
Okay, so if I'm strictly doing GTD it would seem that's a project: take O3 every day during April, and then each pill I take would be a calender item. However, that's horribly inefficient, especially if there are many daily things like that.
It's easy enough to do a workaround, like having a "do every day at home" list or some such, or even having an item in the action list in the right context that stays until the period is over (that's how I sometimes do with reading books, no point in having a book to read as a project and then fiddle with adding a next action after each reading session, that'd be pointless).
However, those solutions are not really GTD as far as I can tell. Is there a strict GTD way to deal with recurring actions that's not too inefficient? If not, it's that a gap in the GTD system? It looks to me that recurring actions like the above are a class of items or actions that aren't accounted for very well in GTD, but I'm not sure. If so, maybe that should be added -- an RA-list of some sort.