All of the Vet I know also blew off Vitamin A -- It is too easy and not expensive enough to make them any money -- at least on the rain rot.
BUT -- it will work 100% of the time and is the cheapest, easiest and quickest way to get horses over rain rot, runny goopy eyes, poor heat cycles, lice and scruffy hair coats that come in the winter and early spring and are seen in poorly fed and debilitated horses.
It can be cheaply ordered from any Catalog farm supply like Jeffers and United Vet and can be cheaply bought at any farm store or feed store that caters to cattlemen.
I have been using nothing but Vitamin A instead of topical treatments for more than 40 years and learned to do it from a Nutritionist with a PhD in livestock nutrition. I have told literally hundreds of people and Vets about it and 100% of them have said it worked better and more quickly than anything they had ever used or prescribed. -- But, suit yourself. They get over it all anyway when they start getting plenty of green grass -- which corrects the Vitamin A deficiency without anyone's help.