OPTI and lactose
I have had milk/lactose issues since birth. If I have milk, it needs to be something other than cow. I can drink some 0% (less lactose in it, which is what I drank as a baby), but not a lot these days. I was on Opti for 3 weeks, and didn't have any problems with it.
If you are really questioning it, see if you can get one to try and see how you react to it.
Cathy