Hello and welcome. My name is ***** ***** I'm a biologist with over 40 years of experience raising rabbits. I'm sorry to hear of this incident. How toxic this plant is depends on the amount ingested. If it was a very very tiny amount, perhaps nothing will happen. It doesn't matter if the leaves are baby or mature, but only the total amount consumed. If your rabbit ate enough, you can expect to see diarrhea, appetite loss, lethargy, and in larger amounts, kidney damage. The kidney damage is a result of calcium oxalate in the leaves. Again, it all depends on the amount eaten.
Usually, toxic reactions show up within ten hours. There isn't much you can do on your own, except give activated charcoal if you have some. You can buy it in pharmacies. Mix it with water and syringe feed it. I would also feed him grass hay. The fiber can push the leaves through more quickly. You'll have to make a decision. You can use charcoal and hay, and watch him closely. If any symptoms appear, you'll have to go to an emergency vet right away. The other option is to see a vet now to have the stomach contents lavaged out. This link will take you to a directory of rabbit vets:
If you have more questions, let me know. I hope that whatever you decide to do, your bunny will be fine.
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