When a cancer diagnosis arrives and treatment is about to begin, it's natural for patients and families to feel they should "do something right now to get the body ready." That impulse often lands on high-dose vitamins — a so-called "megadose." Many people, in particular, start taking around 3,000 mg of vitamin C a day, hoping to build baseline strength before treatment. The short answer is that before you begin or continue any such supplement, it is safest to talk with your treating team first.
There are two main reasons. First, there is concern that high-dose antioxidants like vitamin C could work against chemotherapy and radiation. Some chemotherapy drugs and radiation act partly by raising reactive oxygen species inside cancer cells to damage them, and a strong antioxidant could, in theory, interfere with that process. The research on this is still mixed and far from settled — which is exactly why many oncology teams are cautious about high-dose antioxidant supplements during treatment.
Second, high-dose vitamin C can itself be a burden on the body. Oral vitamin C absorption saturates above a certain amount and the excess is passed in the urine, so pushing the dose higher does not raise blood levels proportionally. Instead it can cause stomach upset such as diarrhea and heartburn, may raise the risk of kidney stones in susceptible people, and calls for extra caution in those with weak kidney function or a specific enzyme deficiency (G6PD). Especially with a cancer like rectal cancer, where bowel problems may already be present, diarrhea can itself affect the treatment schedule and how a person feels.
You may be wondering "how many days before chemo is it still okay to take it." In truth, more important than any single cut-off date is telling your team everything you are currently taking. Bringing the product's ingredient label or packaging into the clinic lets them weigh it against the type of treatment, the patient's kidney and liver status, and interactions with other medicines.
The wish to build up baseline strength is a very good instinct. Preparing the body before treatment is actually called "prehabilitation," and what has relatively clear evidence there is not a single supplement but the whole pattern of daily life: a balanced diet with enough calories and protein, regular walking or light strength work matched to your condition, stopping smoking, managing other issues such as anemia or blood sugar, and getting mentally ready. These contribute far more steadily to enduring treatment than a single high-dose vitamin pill.
This article is for general information and does not replace care for an individual patient. Whether and when to take supplements, and the specifics of exercise and diet, should always be decided in consultation with your treating medical team.