Maria Joao Cardoso, the head breast surgeon at the Champalimaud Cancer Centre in Lisbon, Portugal, said that there was no evidence that herbal therapies or creams worked.
If in doubt, it is best not to take anything, she added.
Garlic, ginger and ginkgo pills, for example, can delay the healing of skin wounds when breast cancer spreads, she said.
"Doctors need to be more proactive about asking their patients what else they are taking when they are being treated for cancer," Cardoso told the BBC.
She said that it is particularly important that patients always check with their doctors first before trying complementary therapies for cancer that has spread to the skin. This happens in one in five cases of breast cancer, and less in other cancers.
The danger is that many products can interfere with the hormone therapy or chemotherapy treatments, and certain ones prolong the blood clotting process, which can lead to wounds taking longer time to heal and more scarring.
She said that herbal products like green chiretta, feverfew, garlic, ginkgo, ginseng, hawthorn, horse chestnut and turmeric slow down clotting.
Cardoso said that it is not surprising that patients and their carers go searching for complementary or alternative treatments that might make a difference.
But she said people should know that "they could end up doing more harm than good".
"The highest goal in medicine is important to remember: Do no harm," she said.
As per the website of Cancer Research UK, some complementary therapies might stop conventional treatments working as well as they should.
( With inputs from IANS )