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Hello. Sorry about the long wait.
Risperidone can cause anything from agitation to muscle tremors to seizures in dogs. It takes effect quite rapidly. Most dogs will show signs within 3-4 hours. If you don't see anything in that time, then he probably won't show anything.
If we see tremors or seizures, we can give Valium to control it and start IV fluids to flush out the drug. With just one tablet ingested, I would be surprised if you see anything more than just some mild agitation. Those mild signs can be ignored and you can let them resolve on their own.
I hope this helps, let me know if you have any other questions.