Vitamin C also works as an antihistamine. It works best for us in our family if we take it with niacin (it just has to be taken in the same day, not at the same time of day): I think of this combination as the dynamic duo because it can do some incredible things. Regular niacin will give you a flush which feels like you have a sunburn for about 1/2 hour if you take enough and if you don't like the flush you can take niacinamide instead.

I have found that this works for colds and my husband uses them for his asthma and I have seen some sites online where people have said that vitamin C has helped their sinus problems, but I think you would be more likely to have good results with both niacin or niacinamide along with the vitamin C if you decide to try it. I don't really know why the niacin helps but it does. For best results you need to find out what your bowel tolerance for C is and then take just under that amount and you have to take it at least twice a day because your body excretes it so fast. You would probably have to take at least 3 grams two or three times a day for good results. You can look up vitamin C bowel tolerance online for more information. And 250 to 500 mg of niacinamide twice a day would probably work OK.