A lot has been said in praise of my good friend Mr. Vitamin C. So it is not a new thing and surprising that it is also very effective in treating allergic reactions like hay fever.
As such i personally recommend taking Vitamin C in the form of ascorbate rather than ascorbic acid. Taken Vitamin C in form of ascorbic acid can lead to gastro-intestinal disorders.

Here is what to do to fight an allergic reaction;

• Consume up to 8 grams, 5 grams are necessary, on average. 
• Start with a dose of 3 grams. 
• Take another 1 or 2 grams every three hours, until the symptoms disappear completely.