Professor David Wilson, from Edinburgh University, said cancer departments did not routinely ask patients about use of complementary and alternative medicine but should since they can affect treatment.
Vitamin supplements containing selenium could worsen toxicity when mixed with chemotherapy agents, while echinacea – made from coneflowers – could affect how the body metabolises drugs, Wilson said.
His warning came after a study led by the university found 55 per cent of Scots children undergoing cancer treatment were also using therapies such as vitamins, minerals and fish oils.
Previous studies have suggested the use of complementary and alternative medicine (CAM) among child cancer patients in the United Kingdom ranges from 33 to 40 per cent, with aromatherapy, massage and multivitamins most commonly reported.
The new study, published in the journal Complementary Therapies in Clinical Practice, questioned 74 families in south-east Scotland with children having cancer treatment.
The results suggested higher use than previous studies, with 53 per cent using vitamins and minerals, 29 per cent massage and an equal proportion using fish oils.
More than a quarter (27 per cent) of CAM users sought private and often expensive therapies such as chiropractic, naturopathic and reflexology.
The most affluent families were almost five times as likely to use CAM as the more deprived ones, reflecting greater awareness and cost concerns.
Respondents said they used therapies in a bid to cut stress, deal with side-effects from treatment and to boost the immune system.
The study, funded by the Fergus Maclay Leukaemia Trust, found use of CAM was perceived to be positive by the families questioned. The researchers said CAM had the potential to improve the quality of life of cancer patients by reducing stress and alleviating symptoms.
But they pointed out many therapies, particularly those of biological origin, could interact with cancer treatment increasing toxicity or reducing their effectiveness.
They also reiterated it was vital patients did not stop their child’s conventional treatment.
Wilson, from the university’s Department of Child Life and Health, said: “The multivitamin antioxidant tablets are very popular.
“Vitamin E is fat-soluble so if you take it in excess it builds up in the body and has been associated with an increasing risk of bleeding. There is also a trace element called selenium, which you need in tiny quantities, but that can worsen toxicity with one of the chemotherapy agents.”
Wilson stressed that there were many positive benefits to using CAM, to complement rather than replace conventional treatment, and the research team had not found any patients suffering as a result.
But he said it was important doctors were aware when they were being used in case something unexplained was happening to their patient.
He said: “We need knowledge that they are on it because there are potential side effects. It should be part of routine clinical practice when checking the drug list with patients to ask if they are on any other supplements.”
Dr Kat Arney, science information manager at Cancer Research UK, said: “It’s understandable that families will want to do anything to help their child get through cancer, and this research shows that some people feel that certain complementary therapies are helpful alongside conventional cancer treatment.
“But there’s little known about how some of these therapies – such as vitamins or herbal supplements – might react with medical treatment, so we recommend that anyone considering complementary therapies first talks it through with their doctor to ensure that it is likely to be safe.
“We would strongly advise people not to take alternative therapy instead of conventional medicine, as there’s no solid evidence that alternative approaches improve cancer survival.
“The treatments currently used for cancer are all evidence-based and have been rigorously tested in clinical trials and lab tests for safety and effectiveness before being given to patients.”