vomiting, or becomes very lethargic take him to a veterinarian
immediately--this could be life threatening.
Ideally this dog should see a veterinarian because injectable treatment
will work faster and is more effective.
At home you can try giving Benadryl at a dose of 1 milligram
per pound of body weight given by mouth three times a day for 2 days. Do NOT give Cold and Sinus variety--only
plain Benadryl is safe.