For a dog, this will work every time, but it is off label usage.
Get some injectable Ivermectin from the vet that is designed for deworming cattle. For a "big" dog, the dose is between 2 and 5 cc. It works on fungus, lice, worms, ticks, fleas--just about anything that grows or bites.
One shot and done.
Ivermectin has no effect on a fungus like ringworm. It's strictly for parasites.
http://link.springer.com/article/10.1007%2Fs10311-003-0049-0
Nope, it cures ringworm just fine, a fungus. Many large animal vets will tell you the same thing on cattle on higher application rates. It's simply off-label use for ringworm. How it works I do not know, but that it does work is a guarantee and rather well-known for many years in farm/ranch country. Heck, I remember the cow vet using it on our dog back when I was kid, so we're talking at least three decades of knowing about it. Maybe it causes some other reaction in the animal that then results in getting rid of ringworm--bottom line is that the critter will be rid of their fungal infection, whether cow or dog.
It works on cats with ringworm too, if anyone cares about them. Don't quote me, but I think the cat dose is 1/2 cc.
If you give too large of a dose to a dog or cat, yes it will kill them.
