It's synthetic progesterone, so like the mini-pill (but not - it does not have SuperContraceptivePowers by itself/in low doses). It's much less prone to Pill-like effects because of the low dose/short time span it's usually prescribed over. It wears off pretty much the day you stop taking it (in August 2002 I stopped it on a Friday, appx. 6 weeks after my last period, and it started Saturday afternoon).