Used Winstrol on a Paint gelding circa 1980. He was no rescue but was poorish, not a big eater. Did not appear to have anything wrong, just been on poor diet too long before I got him.
It worked, picked him right up. Didn’t have any side effects but we only used it a few of times, think it was once a month for 3 months- long time ago. I had no intention of showing him until his condition improved considerable, no idea if it was permitted-kind of doubt it was allowed. Don’t know why you’d want to.
Cant imagine selecting that particular product to somehow enhance a show horse with so many other choices now on the market. Although if I had one that was otherwise healthy but needed a boost to restore appetite after illness or surgery, I might consider it. Short term, no showing.