Levonorgestrel
"I didn't know anything about Mirena except that it was a little T-shaped piece of plastic that lasts for 5 years.  So I chose to get it. I've had slight occasional cramping and rare bleeding that's very light and only lasts a few hours when it does occur.  I've had it in for about 2 1/2 years.  The only scary thing is reading the horror stories all these women have about it.  I have had no problems since it's insertion aside from terrible cramping for about a half hour while my cervix adjusted to it once it was put in.  I know that as long as nothing bad ends up happening that I'll be having it replaced once my 5 years is up.  I love Mirena."