I'm skeptical about calmers tbh. And this is as someone who used to sing the praises of Naked Horse Buck Off and credited it to transforming my horse. I took him off it and it turns out it didn't actually do anything and it was likely just more work and me finding my confidence that has actually transformed his behaviour!
I do think though if they have a placebo effect which means you have the confidence to do more as you 'believe' they are working (in my case) then there's no harm in it.
I would say looking at the ingredients you could either get the ingredients separately and mix your own cheaper or feed Vervain which seems to be the main ingredient.
Did you know that years ago it was common to put a sprig of Elder in your pocket if you were breaking in a horse - to keep him calm! Good placebo effect.
I tried it and didn’t see any results no. It smells amazing but if you look at feeding straights of each herb the recommended dose is way higher than can surely be in the daily amount I think.
We used it on Ozzy when he was being particularly ratty last year; I did feel that it improved him and certainly took the edge of his anxiety. He seemed to love the taste of it too.
I feed their Golden Oldies supplement to Snoopy and it's really good - the difference is obvious.