I do micro chipping so have a sharps box, my micro chip needles and the needles from the any of the animals all go into the box.
I had a word with the vet and he says when the box is full just take it in and they will dispose of it with their stuff.
I was told when I did the micro chipping course that if you take your sharps box to any drug related drop in centre without mentioning what is in there they will dispose of it and give you fresh needles and box

I don't think I would have the courage to try that though
