I used to agree with Speed Racer's view above about 'calming supps' & still do to a degree. I think it is most often about the way the horse is dealt with.... & sometimes it would be best for the handler to be the one taking 'calming supps'!
BUT I have learned & found that regardless of a conventionally well balanced nutrition, horses are very often deficient of magnesium & too high in potassium & among other effects, both these nutritional problems exacerbate 'nervous' behaviour. Ensuring adequate Mg(most calming supps I've seen include Mg) & that potassium levels are kept low, &/or enough sodium in the diet to balance K. does tend to have a positive effect on behaviour/stress.
I agree with Loosie, I used to think calming supplements were a waste of money but after having a horse deficient in magnesium, a calming supplement worked until we found a Smartpak that fit his needs.
I also agree that a horse with either enough turnout or enough exercise to get rid of excess energy and proper nutrition should not be overly nervous.
Nervousness can come from other sources such as anxiety from prior rough/bad handling. I've had a few of those come through my door, but most typical nervousness just comes from either anxiety, imbalanced nutrition, and/or exercise.
Calming supplements do work in the right situation.
My mare's Mg/chromium supplement that she gets for a metabolic issue, Quiessence, is marketed as a calming supplement. I wouldn't give one with many more ingredients than that. Some of the herbal ones may have long term ill effects. For example, many of them have valerian, which has been documented to cause liver damage with long term use/overuse in people. It's perfectly safe every now and then, but is not good when given for months at a time. Many of these herbs have very little in terms of safety data available, particularly for horses.
I'll also add that the "calming" supplement doesn't do much for the little witch's attitude, although it does definitely help the metabolic issue. All horses are different though. You're not likely to hurt a horse with a magnesium supplement at least.
^^Very valid point about herbal supps. Do you know what it is in Valerian that causes liver damage?
Of course absence of proof is not proof of absence either & herbs & such can be incredibly good IME(the herbal mix people around here use for Cushings seems very effective, for the ones that respond to it). BUT people tend to disregard potential for 'side effects' of 'natural' stuff, tend to assume that 'natural' means safe. Arsenic, copper sulphate & selenium for eg are a few natural substances which are deadly...
I can try to look it up. I'm pretty sure that came from the Botanical Safety Handbook, which probably has a reference to the study. It's a pretty good book to have around if you want to play with herbs. I think people tend to forget that plants make some pretty nasty poisons...
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.