Hi Jon ,
Welcome back to the forum.
The use of the Horse product on dogs would be something I just do not have enough information to give an accurate answer n dosage .
However , I will try to answer your question. You will need to decide if the information will benefit you.
Our safeguard information here on Beagles Unlimited is 100 mg. (fenbendazole) per 5 pounds of Beagle Dog weight.
Source of info: http://www.beaglesunlimited.com/health/canine-intestinal-worms-and-inexpensive-treatment
So. for a 20 pound dog you would need a dose of 400 mg. (fenbendazole) per day for 3 days. You will need to look at the dispensed amount of active ingredient per increment on the syringe to get close. If what you have is like the Safe guard 10% syringe I looked at , the amount of active ingredient dispensed is aprox 250 mg (fenbendazole) per notch on the plunger after you purge the air. So , my guess would be 2 notches on plunger for a 20 pound dog per day for 3 days , and , 4 notches on plunger for a 40 pound dog per day for 3 days will get you in the ball park of safe dosing.
Please double check for yourself: http://www.drugs.com/vet/safe-guard-equine-dewormer-25-g.html
For the record , I use the liquid safe guard 10% for my 10 rescue dogs.
Hope this helps.
Best , oldhounddog
