Copper
"I had paragard for a year and a half and had cramping the whole time with terrible long heavy periods.  The last couple months I had it I was in so much pain I could hardly walk. Went to my doc and she said nothing seemed wrong.  Then the next day I went to the ER and had it removed. It had slipped a little and was causing a lot of discomfort. I will never get an IUD again. I'm lucky I didn't need surgery like some women"