Here’s a conversation that happened in my oncology office:
“I love my green drink,” I told my oncologist. “I’ve been having it for years. Can I keep drinking it during chemo?”
She looked at the ingredient list. “There’s a lot of vitamin C in here. That’s not ideal with your chemotherapy.”
“Wait,” I said. “I thought vitamin C was good for cancer?”
Welcome to the vitamin C paradox.
The Dosing Makes All the Difference
Most people think vitamin C is vitamin C. You take a pill. You drink orange juice. You get vitamin C, right?
Not even close.
“It’s two totally different things,” explains Dr. Lise Alschuler, naturopathic oncologist. “When you take vitamin C, your body, most people can’t really utilize more than about 500 milligrams at a time. Even that’s pushing it—probably it’s more like 250 milligrams.”
So that 1,000 mg vitamin C tablet you’re taking? Your body absorbs maybe a quarter of it. The rest gets eliminated, possibly giving you “a little bit more regularity in your bowel movements,” as Dr. Alschuler diplomatically puts it.
But intravenous vitamin C? Completely different ballgame.
“People with intravenous vitamin C are typically getting minimum 20 grams—that’s 20,000 milligrams—up to 75,000 milligrams,” she explains. “Very different dose than that 250 milligrams or 500 milligrams.”
That’s not just a bigger dose. It’s a different mechanism entirely.
How Oral Vitamin C Works
When you take vitamin C orally—whether from food or supplements—it works primarily as an antioxidant. Whatever is absorbed is primarily used to augment the immune system’s ability to attack incoming virally infected cells, or it’s being used to augment the body’s antioxidant defense system.
That’s good for your immune system. It’s great for general health. But it can be problematic with certain chemotherapies.
Some chemotherapy drugs kill cancer cells by generating massive oxidative stress—essentially overwhelming the cancer cell with damaging molecules. Cancer cells are less equipped to handle this oxidative assault, so they die preferentially.
“Theoretically, if you have a lot of extra vitamin C antioxidant floating around, it could interfere with that cytotoxic effect,” Dr. Alschuler explains. “So that’s in essence kind of the argument against oral vitamin C.”
It’s not every chemotherapy. But for regimens that work as oxidants, oral vitamin C could potentially reduce effectiveness.
How IV Vitamin C Works
Here’s where it gets fascinating.
When ascorbate is administered intravenously, the tight controls are bypassed, and pharmacologic millimolar plasma concentrations of vitamin C can easily be achieved. A phase I clinical study revealed that ascorbate concentrations could safely reach 25-30 mM with intravenous infusion of 100 g of vitamin C, with plasma concentrations around 10 mM sustained for at least 4 hours—sufficient to kill cancer cells.
At those high concentrations, vitamin C stops acting like an antioxidant. It flips.
“At that dose, the way that the vitamin C works in the body is it actually acts like an oxidant itself,” Dr. Alschuler says. “So it’s kind of like a natural chemotherapy, not as strong.”
Instead of protecting cells from oxidative stress, high-dose IV vitamin C generates oxidative stress—specifically at the level of the tumor. These high concentrations result in unique chemical reactions within cancer cells that render the cell more vulnerable to chemo- and radiation therapies.
“In that sense, it can be actually quite synergistic with some of these even oxidative chemos,” Dr. Alschuler notes, “because it’s not an antioxidant anymore, it’s generating this what we call tissue peroxide. So it’s kind of adding to that oxidative stress, especially at the level of the tumor.”
The Scientific Research
For decades, vitamin C for cancer was considered quackery. That’s changing.
A randomized phase 2 clinical trial published in 2024 showed that adding high-dose intravenous vitamin C to chemotherapy doubled overall survival of patients with late-stage metastatic pancreatic cancer—from 8 months to 16 months. Progression-free survival extended from 4 months to 6 months.
Let that sink in: doubled survival in one of the deadliest cancers.
“When we started the trial, we thought it would be a success if we got to 12 months survival, but we doubled overall survival to 16 months,” said Dr. Joe Cullen, the University of Iowa researcher who led the study. “The results were so strong in showing the benefit of this therapy for patient survival that we were able to stop the trial early.”
And the patients didn’t just live longer—they felt better. “Not only does it increase overall survival, but the patients seem to feel better with the treatment. They have less side effects, and appear to be able to tolerate more treatment”.
The National Cancer Institute now acknowledges that IV vitamin C combined with other drugs shows promise, with multiple clinical trials ongoing and some showing prolonged overall survival and progression-free survival without added toxicity.
What About the Body of Research?
“Admittedly the body of research is not what an oncologist would characterize as very strong,” Dr. Alschuler says honestly. “But there are some clinical trials that demonstrate some improved efficacy when IV vitamin C is used concurrently with conventional treatment and definitely better toleration.”
Studies show that IV vitamin C may improve quality of life and symptom severity in cancer patients, with improvements observed in pain, fatigue, lack of appetite, nausea/vomiting, and sleep disorders.
A systematic review found that IV vitamin C does not appear to increase toxicity or interfere with antitumor effects of chemotherapy combinations, and may improve time to relapse, enhance reductions in tumor mass, and improve survival when combined with chemotherapy.
The Practical Reality
So should you do IV vitamin C during cancer treatment?
The research is promising. The safety profile is good (for most people—see cautions below). But it’s not FDA-approved, which means insurance won’t cover it, and all batches and brands may not be the same since the FDA does not regularly review how supplements are manufactured.
“It does appear to improve people’s energy and kind of their vitality as they’re going through treatment,” Dr. Alschuler notes.
Here’s what we know:
For pancreatic cancer: Strong phase 2 data showing doubled survival
For other cancers: Promising but less definitive data
For quality of life: Consistent improvements across studies
For safety: Generally well-tolerated in clinical trials
Important Cautions
IV vitamin C is NOT safe for everyone. IV vitamin C may cause serious side effects in people with kidney disease, G6PD deficiency, or hemochromatosis.
You must work with a qualified practitioner who:
Tests for G6PD deficiency first (a genetic condition where high-dose vitamin C can be dangerous)
Monitors kidney function
Coordinates with your oncologist
Uses pharmaceutical-grade vitamin C
Follows proper dosing protocols
The Bottom Line
Oral vitamin C and IV vitamin C aren’t just different doses of the same thing. They’re functionally different tools:
Oral Vitamin C (food or supplements):
Works as an antioxidant
Supports immune function
May interfere with oxidative chemotherapies
Affordable and accessible
Discuss with your oncology team
IV Vitamin C (high-dose, clinical setting):
Works as a pro-oxidant at tumor sites
May enhance chemotherapy effectiveness
May improve quality of life and reduce side effects
Requires medical supervision
Not covered by insurance
My oncologist reviewed my green drink ingredients and said the vitamin C level was “tolerable” with my specific chemo regimen, but asked me to watch it. For other patients with other regimens, the answer might be different.
That’s why you can’t crowdsource cancer treatment decisions on Facebook.
“You think it’s vitamin C, it’s vitamin C,” Dr. Alschuler says, “but it turns out the delivery mechanism apparently matters a lot.”
It matters a lot.
Resources:
Find qualified practitioners: ONCANP.org
University of Iowa Vitamin C Research (search: vitamin C cancer research)
Note: This article is for informational purposes only and is not medical advice. Always discuss any supplements or complementary therapies with your oncology team before starting them.