Supplements & Cancer - What's safe, and what's not?

July 07, 2026•7 min read

The supplement industry is big business.

In the UK alone, the industry is valued at nearly £2 billion and is projected to grow further, driven by a focus on preventative healthcare.

Many people take dietary supplements in the belief that they will improve their health in some way or prevent specific diseases, such as cancer. However, there is no reliable evidence that dietary supplements can prevent, cure or control the growth of cancer.

The World Cancer Research Fund (WCRF) recommends that people don’t use dietary supplements for cancer prevention but instead, focus on the cancer-protective effects of a healthy diet, with nutrients coming from real foods such as wholegrains, fruits and vegetables.

It is recommended people only take a dietary supplement if they are deficient in a certain nutrient and cannot obtain it in sufficient quantities in their diet.

In this blog we explore the use of dietary supplements during cancer treatment.

Should I take a multivitamin supplement during cancer treatment?

If you are eating well, you probably don’t need to take a multivitamin & mineral supplement during cancer treatment.

However, if you are struggling to eat well during treatment, you may not be getting all the nutrients you need from your diet and may benefit from taking a daily supplement. In that case, choose an A-Z multivitamin & mineral supplement that has no more than 100% RNI (Reference Nutrient Intake) for all nutrients (available over the counter from pharmacists and supermarkets).

If you struggle to swallow whole tablets, chewable options are available (e.g. Centrum Fruity Chewables or Superdrug Chewable A-Z).

There is no ‘one size fits all’

Some single nutrient supplements may be beneficial if your blood results show a sub- optimal level or deficiency:

Vitamin D

Vitamin D supports our bone health, immune system, energy levels and mood. It is difficult to get enough vitamin D from diet alone - the main source of the vitamin is skin exposure to sunlight.

A 10mcg (400IU) daily vitamin D supplement is recommended for everyone in the UK during winter months - generally October to April. Some ‘at risk’ groups (e.g. over 65’s, the housebound, and people who cover their skin when outside) should consider taking a 10mcg daily supplement all year round.

Note: for many of us, 10mcg (400IU) isn't enough to keep our vitamin D levels in the optimal range of 75-100nmol; 25mcg (1000IU) is often a better maintenance dose that we see in clinical practice, although this isn't in the public guidelines yet.

Caution: high dose vitamin D supplements can cause serious side effects – more is not better!

Our recommendation is to get tested to know exactly what you need! You can ask your doctor to perform this test, especially if you're struggling with fatigue, aching bones, and low mood. Or there's a few places that offer home testing kits:

BetterYou
Medichecks
Healf - the "core" provides a comprehensive nutritional panel

Iron

Iron is important for our energy levels, immune system and cognitive function. If you have low iron levels, you may be advised to take an iron supplement.

Some iron supplements (e.g. ferrous sulphate and ferrous fumarate) can cause gut irritation and may not be well tolerated. If this is the case, talk to a health professional about taking a chelated iron supplement (e.g. ferrous bisglycinate) which is absorbed more efficiently and less likely to cause constipation or abdominal cramping.

Top tip: take iron supplements with a glass of orange juice containing vitamin C which aids iron absorption. Often taking the supplement every other day is better for absorption as well.

Remember that serum iron is just one marker that looks at iron in the bloodstream. To get a true picture of your iron status, there's a complete iron panel which checks ferritin, folate and vitamin B12 as well as a few other markers. You may need different supplements depending on your full iron studies results.

Our recommendation is to get tested to know exactly what you need! Or you purchase a home testing kits:

Healf - the "core" provides a comprehensive nutritional panel

More is not better…

It is important to be aware that some high dose supplements can interfere with cancer treatment:

High dose antioxidants (e.g. vitamin C, vitamin E, coenzyme Q10, turmeric, and selenium)

Studies have shown that high dose antioxidants can inadvertently protect cancer cells and interact with some cancer therapies (including chemotherapy and radiotherapy), making them less effective. They are therefore not recommended during cancer treatment.

Top tip: Eat the rainbow! Consuming a variety of fruits and vegetables of different colours will give you a range of antioxidants in your diet as well as the benefit of fibre, all of which will support your immune system during cancer treatment.

Omega-3 supplements

Some studies have shown that omega-3 supplements can be beneficial in reducing cancer- related inflammation.

However, they can also interact with some targeted cancer therapies, increasing the risk of bleeding and bruising. They may also interact with some supportive medications used during cancer treatment (e.g. steroids) worsening gastrointestinal side effects.

Consult a health professional before using omega-3 supplements during cancer treatment.

Top tip: Eat oily fish (salmon, mackerel, anchovies, sardines, trout & fresh tuna), walnuts and chia seeds, and use rapeseed oil in cooking instead!

What about Natural Remedies?

‘Natural’ doesn’t always mean ‘safe’.

Some herbal supplements can affect how medication is absorbed or processed. This can mean that too much medication is held in the body (causing toxicity) or the body may process the medication too quickly (making it less effective):

St John’s Wort – one of the most extensively studied herbal supplements. If you are already taking this supplement, it should be stopped before cancer treatment as it may reduce the effectiveness of chemotherapy and can also increase adverse skin effects during radiotherapy. It is also known to interact with how your body processes several medications, reducing their effectiveness (e.g. statins and tricyclic antidepressants).
Turmeric supplements – may lower the effectiveness of certain cancer drugs (e.g. tamoxifen used to treat breast cancer). It is best to avoid turmeric supplements during cancer treatment. Enjoy using turmeric spice in cooking instead!
Ashwagandha supplements – a popular Ayurvedic herb that is used for stress, fatigue and to improve energy levels. However, studies have shown it may increase testosterone levels and impact hormone-related cancers such as hormone-sensitive prostate cancer. Consult a health professional before taking it.

Top tip: Download the ‘About Herbs’ app from your app store. This free app, created by the Memorial Sloan Kettering Cancer Center, is a guide to herbal supplements and complementary therapies for both consumers and health professionals. It allows you to browse supplements by name and provides an evidence-based summary for the chosen supplement including potential side effects and drug interactions.

Key Take Aways…

There is no ‘one size fits all’ - what works for someone else might not be right for you. It depends on your diagnosis, type of treatment, and your blood results.
More is not better - some high dose supplements can interfere with cancer treatment, making it less effective and may cause serious side effects. Always discuss with your cancer team before starting supplements during cancer treatment.
‘Natural’ doesn’t always mean ‘safe’ - some herbal supplements can affect how medication is absorbed or processed. This can potentially result in toxicity or the medication being less effective. Always discuss with your cancer team first.

Always seek specialist advice from a Health Professional before you consider taking dietary supplements during cancer treatment.

Our Specialist Cancer Dietitians can support you with:

Safe, evidence-based dietary supplement guidance
Optimising your diet to support recovery and long-term health so that you don’t need an expensive protocol of supplements
Managing side effects and improving gut health during and after cancer treatment