Levonorgestrel
"I believe myself to be very pain tolerant. I got one of these inserted, and it caused me so much pain I nearly passed out. Keep in mind, however, that I have never been pregnant before. If you have not been pregnant your uterus does not have as much elasticity. Also, my uterus was set back in a position that made for a difficult time for my gynee to get to it. (This happens when you have not been pregnant  sometimes) Nurse said she had never seen the gynee actually have to  utilize the ultrasound to insert it. 1 day after insertion, and now I am completely cramp free. Don't let the horror stories scare you.  Ease of use, cost (completely free), and effectiveness of this product is phenomenal. "