These should NEVER be used as they can be deadly!!!!   I only advise a proper fitting Harness.
A lady purchased a 12 week old puppy from me and 2 days later I received the most heart wrenching phone call that the puppy had got his collar caught on the vanity knob while locked in the bathroom. He twisted, flip flopped and squirmed until he strangled himself to death. He should of never been left by himself in a strange surrounding so soon from being introduced to a new home in the first place. But having a collar on killed him.

I have learned of many other deaths because of collars. Here are several from an acquaintance:
This happened to my neighbors little doxie, and today a customer told me it happened to her sisters little dog in the house. My neighbors little doxie stays outside during the day, she got her collar caught on a screw trying to get under the house. She strangled herself to death trying to get free.
The customers sister also got hers caught on the pull knob of the cabinet door in the bathroom. Luckily this lady was home and found her in time. She is OK, but they have taken the collar off her. My neighbors have now taken all collars off of their dogs. Another puppy hung himself on the drapery cord while the family was gone for just a short time. I have read of other dogs hanging on certain types of baby gates. A friends puppy chewed the edge off a vinyl cloth, stick it's head thru the loose edge, turned circles and would have choked if they had not come home in time.  YOU JUST NEVER KNOW OR THINK THESES THINGS WILL HAPPEN TO YOU.

This is why I always advise everyone that is getting a puppy from me to purchase an Iris Pen to keep their puppy in while they are away. This puppy will be safe and out of harms way.  AND NEVER PUT A COLLAR ON A PUPPY OR AN ADULT!!!!