Levonorgestrel
"Earlier today I was just on this site reading review after review, worried sick about whether or not this worked. It did! This sounds dumb but we didn't technically have sex, but things got pretty serious so I of course got paranoid. I had missed a few days of my BC pill before this, and I started my period the day this happened. I took the pill around 40 hours later, and experienced no side effects. I also started a new pack of birth control the night I took plan b. I took 2 pregnancy tests today which were negative, and my period showed up a few hours ago (4 days late)"