A third of European cancer patients are using complementary and alternative therapies, a survey of 1,000 suggests.
Herbs were the most popular among those surveyed
Herbs are used the most, followed by homeopathy and vitamin and mineral supplements, according to European Oncology Nursing Society members.
Given their popularity, governments should rethink the way these treatments are regulated, they said.
Therapists should also be checked more rigorously, the report in the Annals of Oncology journal suggested.
Dr Alex Molassiotis, from the UK's Manchester University, along with European colleagues, surveyed nearly 1,000 cancer patients from 14 European countries.
About 58 different complementary and alternative medicines (CAM) were mentioned in the survey.
Usage rates varied from less than 15% of patients in Greece to nearly 75% in Italy, and averaged at about a third overall.
Patients typically used more than one CAM therapy together, such as a herbal medicine plus homeopathy or relaxation techniques.
Dr Molassiotis said considering the growing popularity of CAM, better regulation was needed.
Therapies used by cancer patients
Vitamins and minerals
"Anybody can call himself or herself a therapist and practice.
"There is not a body to assess the quality of therapists.
"There are societies that therapists can register with, but it is not compulsory.
"There is a need for regulation of what is accepted and appropriate training."
He said there was also a need for clear guidelines on which treatments work for which conditions.
"We have a responsibility as professionals to look at this and be open-minded."
In the UK, osteopathy and chiropractic are already statutorily regulated.
The Prince of Wales Foundation for Integrated Health was given a government grant last year to look into the regulation of other CAM therapies, including homeopathy, aromatherapy and reflexology.
The Foundation estimates that about one in five people are using CAMs for different ailments.
A spokeswoman from the foundation said: "We have got to make sure people feel as safe as they can through regulation and research."
Dr George Lewith, from the of the University of Southampton's Complementary Medicine Research Unit, cautioned that the study was too small to be able to make generalisations about rates of CAM use, but said there was no doubt that regulation was needed.
Dr Bob Leckridge president of the Faculty of Homeopathy, said many therapists were doctors, which meant they were regulated as an individual.
He thought all CAM practice should be subject to statutory regulation.
Dr Sosie Kassab from the Royal London Homeopathic Hospital said: "Therapies such as homeopathy can be useful as an adjunct to conventional care.
"Patients do say they benefit. We need further research into this."
Professor John Toy of Cancer Research UK said: "The National Cancer Research Institute has recently established a complementary therapies development group, showing the medical profession does not have a dismissive view of this topic."
He advised cancer patients to inform their doctors of all medications and therapies they were taking.