Sometimes as a diagnostic, if a medicine will not do any harm, they can prescribe a med and see if the dog responds to it. if he does, that isolates the problem. With sildenafil, a positive response would indicate a circulation problem. If the possible circulation problem could be life-threatening if not treated early, it's possible to go ahead and prescribe the sildenafil and then go ahead with the lab work as well. If it turns out he does not respond to the sildenafil, we know the side effects are not that bad (unless it lasts for more than 6 hours....).
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