I had allergies before I accidentally thought that taking a MegaB complex would tread my "profound B12 anemia."
I took a lot of the middle strength Mega B vitamins, with a lot of water, and in the worst of hayfever seasons I discovered I didn't have any reaction to walking right up to a huge chamisa (wild sage) and smelling the abundant yellow flowers.
It was great!
Now I don't take nearly as much B complex to achieve the same results, but I do start taking extra a month or so before the beginning of Spring pollen time.
And, it works great. Same with kitties, no longer allergic.
I'm sure that not everyone with allergies has them as a result of low B complex vitamins, but... it sure is worth a try if you have allergies.
I LOVE not having allergies any more.