I haven't take it, but I do take other B vitamins (B-6, B-12).  As with any medicine or supplement, I always start with a small dose.  It's better to buy things in liquid form, tabs that can be broken, or capsules that can be opened for more control over dosing.

  You might want to check the common side effects and see if they're anything that resembles your symptoms.  I don't remember what meds you take, but you can also take the vitamin an hour or so after your antihistamines which might keep down any reaction that could occur.  You can check a list of foods that contain a lot of pantothenic acid and see whether or not you react to any of those.

  The one B vitamin that I know can be a problem is niacin, because it can trigger mast cell degranulation.

If your form of vitamin contains Niacin, you could throw yourself into an allergic reaction easily with flushing. Be careful and safe.
hugs
Ramona