Take this with a grain of salt as I am a first year Beek. I am also a Civil Engineer, with post graduate degrees and am used to doing research on thing I don't have a level on knowledge on and/or uncomfortable with. I am also fortunate enough to be married to a Chemical Engineer with professional registration (PE) who has 30 plus years with the EPA. All of that said and a buck may buy you a cup of coffee.
My use of Formic Pro was based upon a need-my hive, (two strong double deeps) after a clean alcohol wash two weeks prior, started to show DFW on a few bees coming out of one of my two hives. These two hives, had already produced almost a hundred pound of honey by mid August after starting as nuc's at the end of March (3-27). We had exceptional flows this year in NJ, never had a dearth and at one time I had 4 supers on one hive, 5 on the other. When I harvested, first time, I left two supers on each hive as they had not been fully capped. With that situation, I looked at my options. I had a pack of Apivar strips from early spring stored and a Varrox wand and a pound bag of OA-neither are recommended with supers in place. After a lot of reading and discussion, (both myself, daughter and my wife), questions posted here and discussions with pro Beeks, the decision was made to go with the Formic Pro. We printed off the instructions and MSD sheets from Formic Pro and followed the instructions-4 weeks later, the hives are back 100%, 100 pound (each) brood boxes and now 7 supers filling on a strong fall flow. It works.
Yes, there were some pretty shocking losses initially, yes, i think one queen (in the impacted hive) stopped laying for a week or so and yes, dead larvae came out of the hive for a week or so. None of this should be a surprise to any user that read and followed the directions. You are treating for a reason, you have sick bees and impacted bees will die regardless-what are your other options? If you don't do it or another treatment, you might as well pick up a new hobby because you hive will be dead shortly. In my case I could have pulled my supers and started an OAV regiment but all the signs at that time showed a strong flow starting. It's funny too, I personally don't care for honey (heathen!!!) and on my end, am more interested in expanding my comb stores for a hopeful expansion of my apiary next spring. The hives are now 2 deeps each with 7 supers filling between them with plenty of new brood, frames from top to bottom covered in bees, banging the aster and goldenrod at least a month from freeze. Harvest and OAV starting shortly to protect the winter bees.
Bottom line, Formic Pro worked as it should, read the instructions (including adding supers for the initial cluster expansion) and expect the obvious.