Technically the necklace shouldn't work as a contraceptive, since the inside of the womb is not "inside your body" much like your digestive tract isn't. Humans are oddly shaped donuts, your skin/intestines/womb/urethra/eyesockets etc. are all the same external surface. The necklace would remove poisons when they get absorbed by your body, not while they're inside your stomach.
If this isn't true, then it should also be the world's best armor since the moment you try to stab someone with a knife, the knife's particles are inside your body and should be destroyed.