Hi, my mare has been on this for the last 6 months and has had no side effects, she briefly came off when i ran out and was unable to get a bottle for a couple of weeks and i really noticed the difference in her.
She is far less 'emotional' when on the Oestress.
I would say it takes a good month before you would see any difference and needs to be really in their system before their first cycle, this is why i started feeding 6 months ago.