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The safety of vitamins and diet supplements

Read about how safe vitamins and diet supplements are as complementary or alternative therapies.

Vitamins, minerals and other nutrients

We need nutrients such as vitamins, minerals, essential fats and amino acids for our bodies to work properly.

Some examples are:

  • vitamins A, C and D
  • minerals – like zinc, calcium, selenium and magnesium
  • essential fats
  • essential amino acids – like phenylalanine, valine, threonine, tryptophan, isoleucine, methionine, leucine, and lyseine
  • some plant compounds (phyto nutrients or botanicals) – like carotenoids, flavonoids, and isoflavones

The best way to get the vitamins, minerals and other nutrients you need is through a balanced and varied diet, with plenty of fruit and vegetables.

You might find that your cancer or your cancer treatment makes it difficult to eat or drink properly. Or your cancer might stop you absorbing some nutrients from your food or drink. So you might have low levels of particular nutrients.

Low levels of some nutrients in the body can make us ill. For example, if you don’t have enough vitamin C, you can develop a condition called scurvy. Scurvy causes bleeding gums, extreme weakness and bleeding under your skin.

Dietary supplements and cancer

Dietary supplements are also called nutritional supplements. Vitamins and dietary supplements come as pills, tablets or a liquid. Some complementary or alternative therapists also use injections of dietary supplements.

You might need to have dietary supplements if you have low levels of particular nutrients. For example, hormone therapy (often used for breast and prostate cancer) can weaken your bones. So your doctor might give you calcium and vitamin D supplements.

Or, your cancer might stop you from easily absorbing nutrients from your food. So your doctor might prescribe a daily multivitamin and mineral supplement.

Many people with cancer use dietary supplements to help fight their cancer or make them feel better. Most people use supplements alongside their conventional cancer treatments, such as radiotherapy or chemotherapy. But others choose to use them instead of conventional treatments.

Having dietary supplements instead of conventional cancer treatment could be harmful to your health. It might greatly reduce the chance of curing or controlling your cancer.

It is important to talk to a health professional if you're thinking of taking nutritional supplements.

Dietary supplements and preventing cancer

There is no reliable evidence that any dietary supplement can help to prevent cancer. But there is evidence that a healthy diet with plenty of fruit and vegetables can reduce your cancer risk.

Some research has looked at whether particular vitamins and dietary supplements can help to prevent cancer in certain groups of people.

One large study in the USA found that giving vitamin E supplements to male smokers reduced their risk of prostate cancer.

It also found that giving beta carotene supplements to men with low levels of it in their diet reduced their risk of prostate cancer. But the supplements had no effect for non smokers or men who had normal levels of beta carotene from their diet. Beta carotene is a substance the body uses to make vitamin A.

Eating foods that contain beta carotene can help to reduce the risk of lung cancer. But taking beta carotene supplements does not seem to have the same effect.

In 2012 a review of 9 studies looked at the effect of various vitamins and minerals on lung cancer risk. It found that for people not at high risk of lung cancer, none of the supplements had a significant effect compared to those taking dummy tablets (placebo). 

But for people at high risk of lung cancer, such as smokers or people exposed to asbestos, taking beta carotene supplements gave a small increase in lung cancer risk.

Dietary supplements during cancer treatment

Most supplements are safe for people to use alongside conventional cancer treatments. But we don't know much about whether some types of supplement could interact with particular cancer treatments.

Get advice from your doctor, specialist nurse, or dietitian if you want to take supplements and are having any kind of cancer treatment.

Possible risks of nutritional supplements

Some dietary supplements can cause skin sensitivity and severe reactions when taken during radiotherapy treatment. 

Some vitamins or minerals could interfere with how well cancer drugs work. Antioxidant supplements such as co enzyme Q10, selenium and the vitamins A, C and E can help to prevent cell damage. So some doctors think this might stop chemotherapy working well. 

Reducing symptoms or side effects

There is no evidence that nutritional supplements can help to treat any type of cancer. But some small studies show they might help control cancer symptoms or treatment side effects. We still need a lot of research into this area.

Several small studies showed that omega 3 fatty acids in fish oil could help to control extreme weight loss due to cancer (cachexia). But larger trials have not found a significant effect.

The Cochrane Collaboration did a systematic review in 2006 (updated in 2009). It looked at the research on people who have used selenium to help reduce side effects from chemotherapy, radiotherapy and surgery. 

It found no clear evidence that selenium supplements helped reduce treatment side effects. They did find evidence that some people accidently took too much selenium. So they recommended that more research was done to find out what dose people should have, and if it helps to reduce treatment side effects.

Dietary supplements and advanced cancer

There is no strong evidence that supplements can help to treat or control the growth of cancer. But some small pilot studies seemed to show that dietary supplements might help to control the growth of advanced cancer for some people.

For example, one study of 41 people with advanced cancer had supplements of co enzyme Q10. They also had a mixture of antioxidants such as vitamin C, selenium, folic acid and beta carotene.

The patients were estimated to live for an average of 12 months, ranging from 3 to 29 months. But they actually lived for an average of 17 months, ranging from 1 to 120 months. So the patients lived more than 40% longer on average than the researchers predicted. The supplements caused few side effects.

The study seemed to show that dietary supplements might be helpful for some people with advanced cancer. But we need more studies to confirm these results.

European Union (EU) laws on supplements

The European Union Food Supplements Directive (The Directive) deals with the regulation of vitamin and mineral food supplements. It was revised in 2011. 

The Directive is a European safety measure designed to help protect the public and governs:

  • which vitamins and minerals you can buy in the UK and other European Union states
  • the ingredients that food supplements can contain
  • the doses in which they can be present in each supplement

Supplements can only contain vitamins and minerals taken from an approved list.

More information about vitamins and minerals

MedlinePlus in the USA has information about many of the vitamins and minerals used as dietary supplements. It gives information about their possible side effects and interactions, and the research evidence on their use in cancer and other illnesses.

Last reviewed: 
24 Oct 2014
  • Oxford Textbook of Palliative Medicine (3rd edition)
    D Doyle, G Hanks, N Cherny and K Calman
    Oxford University Press, 2005

  • Supplemental and dietary vitamin E, beta carotene and vitamin C intakes and prostate cancer risk
    VA Kirsh and others
    Journal of the National Cancer Institute, 2006. Volume 98, Issue 4

  • Drugs for preventing lung cancer in healthy people
    M Cortés-Jofré and others
    Cochrane Database of Systematic Reviews, 2012. Oct 17

  • A systematic review on the role of fish oil for the treatment of cachexia in advanced cancer: an EPCRC cachexia guidelines project
    A Ries and others
    Palliative Medicine, 2012. Volume 26, Issue 4

  • Selenium for alleviating the side effects of chemotherapy, radiotherapy and surgery in cancer patients
    G Dennert and M Horneber
    Cochrane Database of Systematic Reviews, 2006. July 19

  • Improved survival in patients with end-stage cancer treated with co enzyme Q10 and other antioxidants: a pilot study
    N Hertz and RE Lister
    Journal of International Medical Research, 2009. Volume 37, Issue 6

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