I have been feeding it to my horse with SI - it doesn't stop the SI and I am not sure that it does help my boy - I am giving him this summer to make my final decision on it...
Someone I know who has a pony with SI swears by the vitamin B complex tablets that can be bought from Boots - says brewers yeast makes no difference but these tablets do...If given to a big horse they may not work out any cheaper than brewers yeast.....
As Maesfen says you can get BY quite cheaply to see if it works for your horse.....Feedmark do it and if you can get to an event where they are attending (like Badminton), they normally sell their products half price.....