This drug is not exceptionally toxic to dogs, meaning it won't cause organ failure but it can have some side effects.
At a dose of 0.1 mg per pound of body weight, or one 10mg tablet per 100 pound dog, may lead to sedation, or paradoxical hyperactivity (opposite, unexpected, reverse type effect) may occur in some dogs.
At a dose of 0.3mg per pound of body weight or 10mg for a 33.3 pound dog paradoxical hyperactivity is more likely to occur.
Dogs have been reported to ingest doses up to 10mg per pound of body weight and not die.
Signs of ingestion can occur within one hour, so if has been longer than that inducing vomiting is likely no longer a good option. The symptoms should last for up to 12 hours.
Other symptoms that we can see include panting, vomiting, diarrhea, drooling, tremors, incoordination, and at very high ingested doses we can see a decreased heart and breathing rate.
The concern for Roxy is that she is an older girl, and thus may not tolerate the drug very well.
If she ate 1/2 a pill, depending upon the size (5mg or 10mg), then she ingested 2.5mgs to 5mgs.
If she is about 20 pounds (average schnauzer size) then although this isn't normally a truly dangerous drug we will likely see the symptoms listed above, such as being very sleepy or hyperactive. There isn't much that you can do at home once the drug is absorbed. You might also see panting, nausea, and loose stools, as well as possible tremors. If she is a dog that has had seizures in the past this drug can precipitate seizures.
If she has uncontrollable vomiting, tremors, seizures or cannot seem to settle and has a fast heart rate she should be seen at an emergency clinic.
Please let me know if you have any further questions.