Levonorgestrel
"This works! I'm writing this because, for two weeks after taking this I stressed about it being effective. I'm 21, in college, 5'1 and 110 pounds. I don't take birth control because I am not usually sexually active. Long story short, I had a risky one night stand, unprotected. (stupid, irresponsible) I know. I took plan b one step at around 6:30 the next evening, around 15 hours later. I was especially worried because this was right around ovulation time. I had few side effects. Day 1 was at night, gave me a slight headache and made me depressed. Day 2 I felt fine all day but had a few instances of dizziness. About a week in I had mild cramps and slightly sore breasts. Now, two weeks later, my period came, right on time."