Vitamin C is probably one of the longest known, best studied vitamins of all and has been supplemented by pretty much everyone living in the Western world that has ever seen a hospital from inside due to flu, immune system disorders or other sickness. Everyone seems to know that apples, citrons and Co. contain lots of the healthy vitamin and that the famous C is the very first reason these fruits are considered immunity boosters. This resonates in everyday sayings such as “an apple a day keeps the doctor away” which pretty much exists in every language worldwide. With vitamin C being added to increasingly more (health) products, some consumers might ask themselves if at one point it’s possible to overdose on it.
Is Vitamin C Safe?
First and foremost, it's important to note that vitamin C in its natural form is generally quite safe - even when consumed in large quantities.
The human body can only absorb so much of the nutrient before it gets eliminated by your organs naturally; this means you would have to consume an ungodly amount of food for a prolonged period of time in order to overdose on vitamin C (doses 8-50 times the recommended dietary allowance).
This is highly unlikely as your body typically won’t let you ingest such excessive amounts through reactions such as abdominal pain, vomiting or diarrhea.
What About Not Overdoing It Quite So Much?
However, overdosing on vitamin C supplements containing more than 200mg per serving could put strain on your liver and kidneys and possibly cause a significant increase in blood pressure too.
It’s best not to exceed 1–3 grams per day from any source unless specifically instructed by your doctor.  It's also advised that pregnant women avoid taking high doses of Vitamin C supplement unless their doctors advise otherwise since some studies suggest that higher intakes might increase the risk of premature birth. 
How Much Is Too Little Vitamin C?
This being said, don’t in response take too little Vitamin C, though! Assuming the vitamin C you intake through your regular diet (that hopefully contains some salad and fruits) only amounts to around half what you need per day, adding supplemented C makes perfect sense especially in colder months. Up to 2000mg per day are totally safe to take while some people don’t even experience negative side effects at 3000-4000mg. In case of doubt, just increase your own intake slowly and ask your doctor in case you experience any negative downsides.
It’s important to note that as with any supplement, there is still some controversy following the use of high doses of vitamin C. That being said, many scientific studies indicate that such a dosage is relatively safe and effective in terms of boosting overall immunity.  Ultimately, it’s best to consult your physician prior to taking larger doses or if you are considering using Vitamin C for specific medical conditions (such as cancer treatment) - good advice one should consider when tinkering with nutrients regardless what kind.
(1): Hathcock JN et al., Risk assessment for vitamin C. Regul Toxicol Pharmacol 1989 Mar ; 9(2):184-97
(2): Savitz DA et al., Maternal serum level of alpha-tocopherol and risk for preterm birth among nulliparous women residing in southern California AmJ Epidemiol1991 Feb 1; 133(3):194–202
(3 ): Wu CP et al., Protective effects of vitamins on oxidative stress induced by environmental pollutants Free RadicRes 2003 Apr ; 37(4):403-18