How come i have a severe allergic reaction such as anaphylaxis to vitamin pills but not to vitamins through iv fluids?

Aint' easy sometimes: Be skeptical of reactions to vitamins. It's really rare. Get a wise allergist who you trust to really go through your story. Look at it with open eyes. Sometimes causes are not always easily identified (idiopathic). There are a few tests that are helpful, though.

Exipients: Vitamin pills may have dye and fillers but IV preps do not. There are only very few vitamins in IV form and this may make a difference. Some people are allergic to the gel capsules and this is another cause for anaphylaxis. Niacin in vitamin pills may cause flushing which may be mistook for anaphylaxis. Overall , it is unusual to have anaphylaxis to vitamins.