I've always had all of my dogs chipped, and I will continue to do so.
There is that slight risk of "cancer" whenever you put something foreign in the body - since the body may choose to try and remove it. The odds of cancer really happening is so slim. Is it possible? Yes, but I believe the risk of my dog possibly getting loose never to be found are A LOT more likely.
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I'd rather live with the risk that my dog might be one of very very few who have issues with the microchip then possibly slip out the door one day or off a harness and never be found. I think its one of those things that you never expect to happen until it does.. a few years ago my other yorkie Barry got loose when our maintenance guys accidentally let him out. He somehow managed to make it over 50 miles away from my home, far outside the search range of shelters that I happened to consider. If it wasn't for the microchip that the rescue organization had put in before I adopted him I would have lost him forever. We can't account for everything, but I like to prepare for the ones that I can.