We were very careful with the spot on. It has a clear warning on the box not to get near eyes or nostrils. There were 6 places that you put the spot on on. Behind each "knee" in the front, behind each hock on the croup and on the poll. Like most products it tells you to watch for reactions. I've only used this on my quarter horses so far.
We handled it properly and gave the horses hay afterwards to keep them busy. The flies were gone instantly so they had no reason to rub at their legs,etc. They were at peace which I have not found a single product that has been that good. There are many different kinds of spot on. I used Absorbine. This was recommended by the mgr. at my tack store. Near the end of the two weeks is when I first started seeing the flies again, BUT it was a rare few.
We do use the fly spritzers in each run in shed. There are no flies in the run in sheds when we use those.
The initial cost of the battery operated box is around $35. However, we have only had to use one can per year in the sheds and they are cheap and the batteries seem to last forever. We forgot and left them up a couple of years through the winter. The cans were empty, but the box would keep on spritzing air. They just keep on working. We've had the same boxes for almost 7 years.