Antioxidants and Cancer: A Double-Edged Sword

Dr Ghassan Hamad
Sep 5
4 min read

Updated: Oct 2

**When Antioxidants Protect Us — and When They Protect Cancer?**

Antioxidants are often celebrated as health-protecting compounds. Found in fruits, vegetables, herbs, and supplements, they neutralise harmful free radicals, reduce oxidative stress, and help prevent DNA damage. For healthy individuals, an antioxidant-rich diet plays a protective role, lowering the risk of chronic diseases — including cancer.

But the picture changes once cancer develops. In this context, antioxidants can become a double-edged sword.

When Antioxidants Protect Us — and When They Protect Cancer

Before cancer develops: Antioxidants act as a shield, reducing DNA damage and inflammation that could otherwise trigger tumor formation.
After cancer develops: Cancer cells themselves rely on antioxidants to survive. By raising their antioxidant levels (especially glutathione), tumors can protect themselves against the oxidative stress caused by chemotherapy and radiotherapy.

This means that while natural dietary antioxidants are safe in prevention, high-dose antioxidant supplements during active cancer may be harmful, as they can shield tumour cells and weaken treatment.

Vitamin C: Antioxidant at Low Doses, Pro-Oxidant at High Doses

Vitamin C is a particularly interesting example.

Low oral doses (dietary levels): Vitamin C functions mainly as an antioxidant, protecting cells (including cancer cells) from oxidative stress. At this level, it is not considered useful as a cancer therapy and may even protect tumours.
High intravenous doses: When given in very high doses through an IV, vitamin C no longer acts like a simple antioxidant. Instead, it can behave as a pro-oxidant, generating hydrogen peroxide and increasing oxidative stress inside cancer cells. This selective stress can damage tumor cells while leaving healthy cells unharmed.

Several studies suggest that intravenous high-dose vitamin C may enhance the effects of chemotherapy and radiotherapy by overwhelming cancer cells with oxidative stress.

Making Vitamin C Therapy More Effective: “Priming” Cancer Cells

One exciting area of research is how to make high-dose vitamin C therapy work even better. The idea is to prepare (or “prime”) cancer cells to be more vulnerable before the infusion:

Using oxidative stress–inducing drugs: Compounds like artesunate (derived from the anti-malarial artemisinin) increase oxidative stress inside tumor cells. If given before vitamin C, they may leave the cancer cells less protected, allowing the vitamin C–induced oxidative burst to cause more damage.
Dietary strategies: Short-term fasting or restricting certain amino acids (like methionine or cysteine) lowers antioxidant defenses in tumors, reducing glutathione and making them more sensitive to oxidative therapies.
Combination approaches: Researchers are studying how pairing IV vitamin C with chemotherapy, radiation, or ferroptosis-inducing diets can push cancer cells past their oxidative limit.

In this way, vitamin C works best not in isolation, but as part of a combination strategy that weakens tumor defenses first, then delivers a powerful oxidative strike.

The Problem with Antioxidant Supplements in Cancer

Several studies have shown that supplementing with glutathione (GSH) or its precursors (like N-acetylcysteine, NAC) can worsen cancer outcomes:

In animal models, NAC and glutathione supplementation accelerated tumor growth and reduced the effectiveness of cancer drugs such as sorafenib.
In melanoma and lung cancer models, NAC supplementation increased tumor progression.
In a clinical trial, breast cancer patients who took antioxidant vitamins (A, C, and E) during chemotherapy had higher recurrence and mortality rates compared to those who did not.

Takeaway: Antioxidants that are protective before cancer may undermine therapy once cancer is established.

Antioxidant Deprivation as a Therapeutic Strategy

Researchers are now exploring the opposite approach: starving cancer cells of antioxidants to make them more vulnerable. This is where concepts like the ferroptosis diet come in.

Ferroptosis and the Ferroptosis Diet

Ferroptosis is a type of iron-dependent cell death triggered by toxic oxidative stress.
Cancer cells often avoid ferroptosis by boosting their antioxidant levels, particularly glutathione.
Preclinical studies show that restricting sulfur-containing amino acids (like cysteine and methionine) — which tumors need to make glutathione — can induce ferroptosis and shrink tumors. For example, in glioma and pancreatic cancer models, a cysteine/methionine-restricted diet increased tumor cell death and improved survival.

This dietary approach — sometimes called a “ferroptosis diet” — reduces antioxidant availability and tips the balance toward lethal oxidative stress inside tumors.

Targeting the Pentose Phosphate Pathway (PPP)

Cancer cells don’t just import antioxidants — they also make more. One of their key tricks is the pentose phosphate pathway (PPP), a cellular system that produces NADPH. NADPH is essential for regenerating glutathione, keeping tumors well-protected from oxidative damage.

By blocking the PPP:

Tumors lose their supply of NADPH.
Glutathione cannot be recycled effectively.
Cancer cells become more sensitive to chemotherapy and radiation.

In liver and other cancer models, blocking enzymes like G6PD or transketolase in the PPP increased oxidative stress, slowed tumor growth, and enhanced the effects of chemotherapy.

What About Green Juicing and the Gerson Diet?

Many cancer patients turn to intensive green vegetable juicing programs or diets such as the Gerson therapy, which involve frequent consumption of freshly pressed juices rich in antioxidants and plant nutrients.

While vegetables and fruits are healthy in general, the concern is that in large, concentrated amounts, such juicing may provide very high levels of antioxidants. In theory, this could blunt the oxidative stress that conventional treatments (like chemotherapy and radiotherapy) rely on to kill cancer cells.

Potential benefit: Juices may improve general nutrition and supply important vitamins and minerals.
Potential risk: Overloading the body with antioxidants through continuous juicing may protect tumors in the same way that antioxidant supplements do, making treatments less effective.

So while moderate consumption of vegetables and fruits is always encouraged, relying heavily on continuous juicing as a “therapy” — especially during active treatment — may be counterproductive.

The Bottom Line for Patients

Prevention: Antioxidant-rich foods (fruits, vegetables, herbs) are protective before cancer develops.
During cancer: High-dose antioxidant supplements (like vitamin E, NAC, or glutathione) may protect tumors and interfere with treatment. Excessive juicing diets may carry similar risks.
Vitamin C: Small oral doses act as an antioxidant (not helpful once cancer is present), but high-dose IV vitamin C acts as a pro-oxidant and can damage cancer cells. Its effect may be even stronger if cancer cells are first “primed” with oxidative stress–inducing drugs (like artesunate) or diets that lower antioxidant defenses.
Emerging therapies: Strategies such as ferroptosis diets and PPP inhibitors aim to weaken tumor antioxidant systems, making cancer treatments more effective.