Levonorgestrel
"The first time I had this pill was after I lost my virginity in a one night stand and due to circumstances I would rather not talk about contraceptives weren't used. Due to it also being a bank holiday and having no transport I ended up waiting 72 hours (the maximum time) before I could get a hold of this emergency measure; the side effects of this time were very minimal - I felt nauseated and very tired - the medication, however, didn't work and I ended up pregnant (later miscarrying). 
I have taken the tablet again today, less than twelve hours afterward as a precaution (the condom came off inside me) and the side effects have been a lot more severe including extreme dizziness and nausea. I do, however, believe it is working this time."