The pill if taken correctly is 99.7% effective (or that's the most common number i've seen) and it can be less effective if you throw up or have diarrhea. But i've looked this up and a site i've found says that just one bout of diarrhea shouldn't effect it. You need to have it continusously for 1-2 days before it becomes less effective. How less effective, I don't know. I believe it's the same with throwing up (throwing up for 1-2 days before effectiveness goes down) but i;m pretty sure that irregular periods (so long as you keep taking the pill, except on your week off) don't effect it.