couple of weeks I think.....but when I use it in horses, I have to give it again in 3 days and again on the 7th day to really eradicate them.

My question has always been, even though it is still effective, for several weeks, is it strong enough to kill, or is it just allowing them to mutate against it.

For a long period that was really the only thing there was for us to give the horses, you know say 10 years ago, and back then, we only gave it once, now I have to give it 3 times in a seven day period, or I know, I am just waisting my money.

Now this is all horses, but.............