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If he just ate it you will need to take him into the kitchen and give him hydrogen peroxide 2 tablespoons at a time until he vomits. Keep doing it until you actually see him throw up. Search for the pill. If he does not vomit then I suggest contacting your local veterinary ER as they may want to induce vomiting. Clonazepam has been used in dogs to hep control seizures. The dosage he has consumed is actually below the normal dosing range for a dog his weight. Most likely he will have no side effects from consuming this drug at a very low dose however because dogs are very individual you will need to monitor for excessive sedation, weakness, difficulty breathing and vomiting. I do expect you to see some tiredness but what I am talking about is falling over and not able to arose him easily.
Please let me know if further assistance is needed.