IUD/IUS

I’m thinking about getting an IUD/IUS and I had a few questions! I have a boyfriend and we’ve been together for almost a year now so I know we’re safe from STI’s, is it safe to have sex without a condom when you have an IUD? Is it safer than when you’re on the birth control pill? I’m scared about it falling out or it hurting me. My sister had a bad reaction to one and almost died so I am super worried about it. If you guys have any info about them please let me know! Thank you!