When you receive a cancer diagnosis and are waiting to begin treatment, it is natural to want to "do something helpful in advance." One idea that comes up often is megadose vitamin C — doses measured in grams (g), far above the usual daily recommendation. Many people ask whether it is fine to start such high-dose supplementation on their own during the pre-treatment window, before any surgery or chemotherapy has begun.
The first thing to understand is that even at "high dose," how much actually reaches the body depends heavily on the route. Oral vitamin C is limited by how much the gut can absorb, so no matter how much you take, blood levels do not rise past a certain point and the excess is passed in the urine. Intravenous (IV) vitamin C, by contrast, reaches far higher blood concentrations. So the same word "megadose" behaves very differently by mouth versus by vein, and the potential benefits and risks should be weighed separately for each.
The two areas most likely to feel the strain of high-dose vitamin C are the digestive tract and the kidneys. Taking large amounts at once commonly causes diarrhea, abdominal discomfort, or heartburn, and because vitamin C is partly converted to oxalate in the body, it can raise the risk of kidney stones in some people. Extra caution is warranted if kidney function is already reduced or if dehydration is frequent. Vitamin C also promotes iron absorption, so care is needed with iron overload, and it can interfere with certain tests such as some glucose meters and stool occult-blood tests.
When several chronic conditions are present, there is more to check. For example, a history of cerebral infarction (stroke) often means taking antiplatelet or anticoagulant medication, and blood-pressure and cholesterol (hyperlipidemia) drugs are frequently taken alongside. How a high-dose supplement fits with these medicines, and whether it burdens the kidneys, fluids, or electrolytes, varies from person to person — so it is safest to bring your full medication list and go through it item by item with your care team.
The "before treatment" timing itself is an important factor. If the stage and treatment plan are not yet settled, the priority is accurate diagnosis through biopsy and imaging and setting the treatment direction. How high-dose antioxidant supplementation may interact with upcoming chemotherapy or radiation is still an area of ongoing study, so it is wise to align the very start date with your care team. Above all, there is no established evidence that high-dose vitamin C replaces standard treatment, so it is best seen not as a substitute for care but as something to discuss whether or not to use alongside it.
In short, if high-dose vitamin C interests you, rather than deciding to start on your own, use this diagnosis-and-planning window to ask your care team together: whether you may take it, and if so when and how much is appropriate. This article is for general information and does not replace individual medical care; whether and when to take supplements should be decided in consultation with your treating clinicians.