I think the best thing for a spooky/difficult horse is a confident, knowledgeable rider, and consistent work. My horse can be a bit of a brat sometimes, mostly if he hasn't been ridden in a few weeks, but there are also times when I am not in the right mind set and I let my emotions get the better of me. He can tell when I am stressed or worried and he gives it right back to me in the form of spooking at nothing, rushing, or just plain ignoring me all together to save his own skin. Things have gotten tremendously better since I have started taking lesson with him on a regular basis. He is getting worked, and I am becoming a better, more confident rider.
That being said, I do give my horse Vitacalm. He has been on it for about 6 months now and he does just fine with it. He has also been on SmartCalm Ultra in the past, and it was just the same. He went off it for about 5 months and there wasn't a huge change, but I did notice that he was a little jumpier, and a little more distracted by random noises. After I put him back on a calming supplement, he was fine again. He will still occasionally stare at things, maybe jolt a little if something startles him, but he doesn't lunge sideways or spin around like he has done in the past. Maybe he has an imbalance that is corrected with magnesium, like loosie mentioned. It would make sense if I see a change in him when he gets more of it.
The point is, I don't expect calming supplements to be a miracle cure-all, but I like what they do for my horse. He is still his quirky self, just a little more focused on the task at hand. As I said before, I owe a lot to the level of work we have been doing in the past few months, the supplement is just another boost.
"A rider who would trade partnership for obedience
will have to settle for neither."