Can vitamin c cause an allergic reaction of mouth? such as, inflammation of the mouth and/or gums.

Possibly: But it is also can cause sores and inflammed gums if you are taking megadoses (more that is recommended on the bottle).

Not likely: All food, medications including vitamins can cause allergic reaction but the possibility of allergic reaction to vitamin c is unlikely as it is present in most fruits and vegetables. The lack of vit c causes scurvy with bleeding gums. It is an very old disease seen in sailors who eat no vegetables. Mega dose of vitamins can be toxic to your body! see a periodontist for you gum condition.