I use these collars for my own cats and I know several people who use them. In most cases they make calming or reassuring effect - cats become more calm, more sociable and tender or less aggressive.
But in 3 cases cats became apathetic, they slept all the time, didn't want to eat. They licked their collars but there was no vomit or any other alarming symptoms. They just slept much and didn't play. All three are males with idiopathic cystitis. When the collar was taken off for some time, one of cats became more active, and later he behaved almost normally when the collar was put on again. Maybe the collar itself was a stress for them and later they got used to it but this is only a guess.
This is very strange. I would be glad to get a consultation from a vet what happens in such cases.