Is it possible to overdose on vitamin c?

No such thing unless. In general vitamin c is extremely safe and non-toxic and you can not overdose, in the sense of taking a quantity that would damage your health. There is a limit to how much you can absorb, and when this amount is exceeded you won't absorb any more and it will act as a laxative.The exception is people who have a hereditary g6pd deficiency, in which high doses of IV vit c can cause hemolytic anemia.
Not a classic o/d. Taking excessive amounts of Vitamin C can cause symptoms such as nausea, vomiting, loose stools, heartburn, abdominal cramping, bloating,headache and insomnia. It may be associated with renal calculi. It can lead to hemolytic anemia in patients with g6pd deficiency.