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The therapeutic dose of proin is 2mg/kg and the toxic dose is about 48mg/kg. Side effects include vomiting, anorexia, restlessness, anxiety, irritability, urine retention, rapid heart rate, and high blood pressure even at therapeutic doses so its possible that she is experiencing an expected side effect, however since she vomited it is likely that she was able to rid her system of a portion of the drug. If this is the case then the things that you are noticing are likely due to another cause and I would recommend taking her to your vet in the morning for evaluation if she is still breathing heavy. As there are lots of reasons that a 14 year old dog can experience breathing difficulty and a veterinarian exam is the best way to figure out what is going on.
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