We nearly lost our wether on Saurday after he ate rhododendron.
We gave him black tea with sugar, then same again with charcoal ground up in it. Then the vet gave him pen&strep, a steroid, a vitamin injection and some twin lamb drench. We gave him more charcoal at bedtime and more in the morning but by then he was fine.
Coffee or anything with caffeine and charcoal seem the preferred treatment, although the vet said there was no antidote to the poison, it just depended how much the sheep had eaten in relation to its size.