I have and did a bit of research on it, decided we didn't need it as it was for horses in tough work.
It's a high protein formula (bit like what body builders eat in those tubs of protein shakes!) and I guess he is being given it to help repair damaged tissue which could make sense. Easier than feeding a combination of straights like oats or linseed which contain much of the same stuff but obviously comes with it excess energy.
I fed it a couple of years ago to a horse who had been unwell and suffered a major loss of topline - and it did a fantastic job helping the horse to put topline back on - this was a youngster not in work and the Myoplast was fed at vets suggestion. I'd definitely use it again if I needed help with topline in a horse with issues.
Just to add my TB events BE, has a very good topline and no probs putting weight on and looks v well. He's just had a 6 week holiday and is now being brought back into work.
I had my mare on it for a while. Vet said that you needs 6 weeks worth before you tell any difference. She'd gone from box rest though to being worked 5 times a week so was bound to muscle up. I only used it as vet recommended it and as insurance paid. I wouldnt of bought it for the price of it though! x Didnt work wonders i don't think, but the work increase did x
Another girl on my yard used it too after her horse had an accident, she stopped using it as she felt it did nothing x As all supplements though, i guess that they work differently with all horses depending on why they are on it, how much work they do and they they are worked x