Hello, my name is***** and I have over 20 years of experience as a veterinarian. I am sorry to hear that your fellow may have ingested an Acyclovir tablet.
What milligram size was the tablet?
How long ago did he ingest it? (less than or more than 2 hours ago?)
A one time ingestion in a bigger dog may not lead to toxicity but this drug can be toxic if he has ingested above the threshiold amount.
The most common signs of toxicity include vomiting, diarrhea, loss of appetite, and lethargy. Signs usually develop within 3 hours of ingestion. Acyclovir can cause kidney damage and suppress the bone marrow as well as cause esophageal and stomach lining damage.
Treatment includes inducing vomiting, administration of activated charcoal, pushing fluids, and supportive care.
If your pup has ingested more than 10mgs per kilogram (2.2 pounds of body weight) we may see signs of toxicity.
At 60 pounds he is 27kgs, so if he ingested more than 270mgs we should induce vomiting if it has been less than 2 hours.
You can give him 3 tablespoons of 3% hydrogen peroxide soaked up with bread chunks or a small amount of food.
Then take him outside and run him around to get the peroxide bubbling so he vomits.
If no vomiting in 10 minutes repeat the peroxide dose once.
If he vomits and seems to clear his stomach and be done I would check to see if most or all of the pill is present.
I'll be back with more information, but I wanted you to have this to start.