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I had a m31 reject, probably a year after implanting. My finger felt very tender / sore and swollen, like there was a splinter in it. The skin on the finger was very tight, like a balloon that had been blown up a bit. I also felt lightly feverish / clammy all over. I ended up swinging by the injury clinic and had a doctor pull it out - he was accommodating and worked it out with a scalpel and another magnet.
Here's the thread if you're interested: https://forum.biohack.me/discussion/2057/possible-m31-rejection-after-2-years#latest
My first magnet rejected (Haworth magnet in my pinky). A week or two after implantation, it started pushing its way out like a splinter. Started as a bulge in the mostly healed incision site. The bulge got bigger, and eventually part of the magnet started poking through the skin. It pushed out farther until it fell out of my finger, leaving a smooth disk-shaped depression in my finger that healed over quickly. This whole process took about a week, and wasn't particularly painful.