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So the chance of a toxin showing up on bloodwork is actually pretty low. There are literally hundreds of potential toxins that could be considered to cause lethargy and sedation. It could be anything from Xanax to marijuana to Benadryl.
Some toxins will show an increase in liver enzyme elevations. This will occur within 12-24 hours of the ingestion. It may then take 3-5 days for that enzyme elevation to come back down to normal.
I would get bloodwork done within 24 hours of the sedation that you noticed at home. We won't be able to do a tox screen like in people, but we can get an idea if the liver is working harder to rid the body of something.
Most non-fatal toxins will have a dog that is back to normal within 24-48 hours. The really bad ones- antifreeze, Bromethalin rat poison, ricin- those are typically fatal once signs are seen. It doesn't sound like that is going to be the case.
I hope this helps, let me know if you have any other questions.