From what I've heard (including the vet) the collars are basically useless. Before I knew that, I had one on one of my dogs - she got it caught in her mouth and got pretty sick. She shook for over an hour and lost control of her bladder and bowels. She was foaming at the mouth... it was awful. I will never, ever but another one.
I am looking into something more natural this year so I'm not putting poisons into their little bodies.
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As far as side effects from sprays and drops... they basically are a poison and they enter the bloodstream, so how can they NOT have side effects at some point? I know they are so easy to use but since I have found out more about them, I'm kind of leary to use them.