Well-intentioned, but dangerous in wrong doses: dietary supplements as a potential health risk during cancer treatment

“Toxicology deals with substances that can have a negative impact on human and animal health, as well as on the environment. It is important for assessing how hazardous a substance is and is therefore of fundamental importance for the future of us all. Particularly when it comes to assessing our food, the effects of food additives and the interaction between medical treatments such as chemotherapy and our diet, our expertise is essential for health and the success of treatment”, says Prof. Doris Marko, Professor of Food Chemistry and Head of the Institute of Food Chemistry and Toxicology at the University of Vienna, as well as Chair of the Local Organizing Committee for the 2026 EUROTOX Congress.

From food to toxic dietary supplement

‘It is the dose that makes the poison. This also applies to our food. Herbs such as basil taste good and are completely harmless when consumed in a normal portion with tomato and mozzarella. However, when taken as a highly concentrated extract or flavouring oil, the toxicological assessment is quite different, as the body then consumes basil constituents in quantities it would never naturally ingest. Too much of this can then be harmful to health, and may even cause genetic damage,” emphasizes Marko. In particular, mixtures such as smoothies, which contain certain substances that one would never normally consume but which are masked by the flavour of the other ingredients, can have undesirable effects because they disrupt the body’s natural regulatory mechanisms. People also tend to take dietary supplements in tablet form quite readily. “These highly concentrated extracts are dietary supplements, but if they are made from foodstuffs, they are still classified as food and do not require health assessments, as is necessary for medical products, for example,” emphasizes Marko.

Undermining chemotherapy: the dark side of dietary supplements

“The problem now is that, due to their high dosage, dietary supplements can have a negative impact on our health and on certain medical treatments. For example, people undergoing intensive chemotherapy often turn to various dietary supplements out of desperation, hoping they will help them. However, some foods contain phytochemicals which can interfere with substances developed for chemotherapy. In extreme cases, the consumption of such foods – particularly in highly concentrated forms – can exacerbate the side effects of chemotherapy or even undermine the therapy’s effectiveness,” explains Marko, who has been researching this topic for decades and will deliver a keynote speech at the EUROTOX Congress on “The Dark Side of Food Supplements”, during which she will present groundbreaking findings. Chemotherapeutic drugs, however, are potent medicines that are calibrated to a specific therapeutic window, so that they do not become too toxic for the body whilst still achieving the targeted effect of combating cancer.

Soy-based dietary supplements can be fatal in cases of hormone-dependent tumors

Isoflavones – compounds isolated from soy beans that are hormonally active – have long been the subject of heated debate as dietary supplements. They are said to alleviate the symptoms of the menopause in women. “However, if a woman has a hormone-dependent tumor, such as certain types of breast cancer, the treatment specifically aims to lower the levels of female hormones using medication. Isoflavones, whether taken as dietary supplements or used in various creams and tinctures designed to reduce menopausal symptoms such as hot flushes, wrinkles, dry skin and so on, can, in extreme cases, counteract the effects of chemotherapy and thus undermine cancer treatment,” says Marko.

Activated charcoal renders the contraceptive pill ineffective

Certain dietary trends can also have serious consequences when taken with other medicines, such as the contraceptive pill. “There are reports of detox smoothies containing activated charcoal causing the contraceptive pill, when taken with these smoothies, to become ineffective because the activated charcoal absorbed the active ingredients of the pill, thereby blocking their absorption into the body,” explains the toxicologist. This may result in unexpected pregnancies.

Mold and bacteria – toxic substances in food

But it is not only dietary supplements that can become toxic; other foods can also pose a risk, for example when mold develops in them. “Aflatoxins are contaminants frequently found in moldy nuts, oilseeds or even cereals, and are known to cause liver cancer in humans,” says Marko. These compounds were detected already in the 1960s when about 100,000 turkeys died of liver failure in England because of moldy peanut-based animal feed. “Recently, aflatoxins have also been detected in almond-based plant-based milk alternatives, leading to new risk management measures for these products. Pistachios – e.g. used in Dubai Chocolate, amongst others – can also contain aflatoxins and must therefore be closely monitored,” explains Marko. Aflatoxin M1, a slightly modified mold toxin that accumulates in the milk of animals that have eaten moldy animal feed, is one way in which mold toxins enter our food. “The problem with aflatoxins is that they highly carcinogenic but cannot always be recognized as mold by the naked eye,” explains Marko. Whilst aflatoxins are carcinogenic other the mold-derived substances like deoxynivalenol affect the gastrointestinal tract and can cause not only vomiting but also intestinal inflammation. Deoxynivalenol is a substance that is very commonly found in cereals. Deoxynivalenol belongs to the group of so-called Fusarium toxins, which were also found in remarkable concentrations in breakfast cereals in Austria earlier this year. “Just as serious as the contamination of food by molds is contamination by bacteria such as Clostridia in tinned food, which can lead not only to gastrointestinal complaints but also to poisoning that can be life-threatening,” says Marko. Toxicology therefore plays a key role through its contribution to risk assessment.

It’s all in the preparation

However, there are some things that are entirely within our own control. Excessive levels of acrylamide can contribute to the risk of cancer. It is produced when starchy foods are heated to high temperatures – such as during baking, deep-frying or roasting – as is the case with crisps and chips. Here, everyone can decide for themselves how much acrylamide they consume by choosing how they prepare their potatoes and bakeries. Certain dietary recommendations – such as washing rice before cooking – also have a protective effect. Washing the rice can help to flush out arsenic, a natural contaminant that accumulates in the rice during cultivation.

About IAKW-AG and EUROTOX

IAKW-AG (Internationales Amtssitz- und Konferenzzentrum Wien, Aktiengesellschaft) is responsible for the maintenance of the Vienna International Centre (VIC) and the operation of the Austria Center Vienna. With 21 halls, 134 meeting rooms and around 26,000 m² of exhibition space, the Austria Center Vienna is Austria’s largest conference centre and ranks among the leading players in the international conference industry. EUROTOX is the Federation of European Toxicologists and European Societies of Toxicology. Through the EUROTOX Congress, it organizes the largest toxicology congress in Europe and plays a key role in promoting the safety of people, animals and the environment through science and education.