This article says:
Any procedure which is necessary to perform for the patient, but it is clear that it does not at all need to be performed on Shabbat, should be delayed until after Shabbat.
How is "need to be performed" defined? Let's say a person is taking a daily medicine for some condition (and assume taking it involves doing a melacha). Can he take it on Shabbos because of pikuach nefesh concerns? Skipping a dose each Shabbos will not immediately kill him, but it will shorten his lifespan (so you can say it will 'kill him' later).