Welcome to Justanswer! I am Dr. Altman and happy to assist you both today!
This is not a medication used in dogs but there are studies for human use that indicated there was no toxicity observed at 12 months of oral dosing in dogs at maximum doses of 20 mg/kg/day.
So with Tucker being an adult Golden likely >50 pounds I would not expect any side effects. I would monitor for vomiting, diarrhea, lethargy, reduced responsiveness, dizziness (wobbly on his feet), respiratory changes, unable to rest comfortably in the next 4-6 hours as a precaution and up to 24 hours to ensure there are no ill effects if he ingested this medication.
The only toxicity report found was "the oral LD50 of Doxazosin is greater than 1000 mg/kg in mice and rats. The most likely manifestation of overdosage would be hypotension, for which the usual treatment would be intravenous infusion of fluid. As Doxazosin is highly protein bound, dialysis would not be indicated."
Please let me know if I can assist with any additional questions or concerns today. If you want to make absolutely sure there is no exposure and if your dog potentially ingested this within the last hour then you can induce vomiting with 3% hydrogen peroxide 1 tsp per ten pounds then walk around to mix it in the stomach and agitate the lining. If no vomiting within 10-15 minutes it can be repeated once only in hopes of inducing vomiting.
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