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Nutrition and daily life

Vitamin supplements: when they help and when they harm

Many people want to take vitamins "to get stronger" during treatment. Sometimes there really is a deficiency and the doctor prescribes a vitamin. But high-dose vitamins, especially antioxidants, can interfere with treatment.

Last updated: ~1 min read
Bottles in shadow, a colourful plate in light.
Figure 1.Food first — Vitamin supplements: when they help and when they harm

01Short answer

Take vitamins only on your team's advice. Deficiency is found by blood tests, for example of vitamin D, B12 or folate. During chemotherapy and radiotherapy avoid high doses of vitamins C and E and other antioxidants unless your doctor says otherwise. With some leukemia treatments, folic acid (with methotrexate) or vitamin A (with ATRA) can interact with the medicines. Do not take iron on your own while having transfusions, as iron overload can occur.

02Simple explanation

Only on advice.

A test shows deficiency.

No high doses.

No iron on your own.

03Scientific explanation

NIH ODS: there is concern that high-dose antioxidants may reduce the effectiveness of chemotherapy and radiotherapy.

NCCIH: "natural" does not mean "safe"; the contents and quality of supplements are not checked as strictly as medicines.

04What happens in the body?

Vitamin D

Deficiency is common; the level is checked and the doctor chooses the dose.

An egg and milk on a sunny windowsill.
Figure 2.Vitamin D — Vitamin supplements: when they help and when they harm

Multivitamins

A standard-dose multivitamin is sometimes fine; ask your team.

The pharmacist
Figure 3.The pharmacist — Vitamin supplements: when they help and when they harm

Interactions

Supplements can change medicine levels in the blood through liver enzymes such as CYP3A4.

Food first

A varied diet usually provides the vitamins you need; supplements only when there is a need.

05Supplements: four steps

List them, show your team, check the evidence, take only with agreement.

List — Write everything down.1ListWrite everything down.
Show — Doctor and pharmacist.2ShowDoctor and pharmacist.
Check — Evidence and risk.3CheckEvidence and risk.
Agree — Only with approval.4AgreeOnly with approval.
OpennessTell everything.
FoodPlate first.
SafetyInteractions.

06Key points at a glance

TopicIn brief
Vitamin DDeficiency is common; the level is checked and the doctor chooses the dose.
MultivitaminsA standard-dose multivitamin is sometimes fine; ask your team.
InteractionsSupplements can change medicine levels in the blood through liver enzymes such as CYP3A4.
Food firstA varied diet usually provides the vitamins you need; supplements only when there is a need.

07Key terms

Antioxidant
A substance that neutralises free radicals.
Iron overload
Iron building up after many transfusions.
Dietary supplement
A product of vitamins, minerals, herbs or other substances.
Interaction
One substance changing the effect of another.

08Key points

  • Rely on tests.
  • Avoid high doses.
  • No iron on your own.
  • A supplement is not a treatment.
  • Report every one.

09Frequently asked questions

Does vitamin C protect against infection?

High doses do not, and they may interfere with treatment.

Should I give my child vitamins?

Only on the advice of the paediatric haematologist.

Is a "natural" supplement harmless?

No. Natural substances can also act strongly and interact.

10Questions for your doctor

Take these questions to your appointment. The ones you save are collected on the “My questions” page and stay on this device only.

  1. Does vitamin C protect against infection?

  2. Should I give my child vitamins?

  3. Is a "natural" supplement harmless?

My questions

12Sources

  1. NIH Office of Dietary Supplements. Vitamin D Fact Sheet
  2. NIH Office of Dietary Supplements. Dietary Supplement Fact Sheets
  3. NCCIH. Using Dietary Supplements Wisely
  4. National Cancer Institute. Complementary and Alternative Medicine
  5. Memorial Sloan Kettering. About Herbs, Botanicals and Other Products
  6. National Cancer Institute (NCI). Eating Hints: Before, during, and after Cancer Treatment
  7. Leukemia & Lymphoma Society. Diet Guidelines for Immunosuppressed Patients
  8. World Health Organization (WHO). WHO guidelines on hand hygiene in health care
  9. Memorial Sloan Kettering Cancer Center. Hand Hygiene and Preventing Infection
  10. NHS. Food poisoning
Medical disclaimerThis information is for educational purposes only and does not replace medical advice.
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