Short answer, not entirely functional until you've been on them a full month.

Long answer/health lesson (sorry...but even if you're not interested, someone else may be): There are two mechanisms by which most oral contraceptives prevent pregnancy. First, and most importantly, it prevents ovulation. No egg = no chance to get pregnant. For that to work, though, you have to be taking the pills well before you ovulate. That's why they say you're not safe the first month....if you didn't start soon enough, you didn't prevent ovulatation. The other half of the effect is to make implantation in the wall of the uterus less likely IF ovulation happens (usually because someone oopsed and missed pills) and IF fertilization then takes place (kind of a belt and suspenders approach). So you are partially protected even in that first month. But like Englishlush said, if you have reason to worry about the results if the ammo turns out to be live...you shouldn't jump the gun (how's that for a thoroughly mixed metaphor ). One more month of condums is much less hassle than 18 years of everything elses, in my always humble opinion .