Acai berry, cumin, herbal tea, turmeric and long-term use of garlic—all herbal supplements commonly believed to offer health benefits—may negatively impact chemotherapy treatment, according to a report presented at the American Society of Clinical Oncology (ASCO) meeting.

Researchers from Northwestern Memorial hospital said there is growing evidence these popular supplements may intensify or weaken the effect of chemotherapy drugs and in some cases, may cause a toxic, even lethal reaction.

Herbal supplements, defined as plant or plant parts used for therapeutic purposes, can interact with chemotherapy drugs through different mechanisms. Some herbs can interfere with the metabolism of the drugs, making them less effective while other herbs such as long-term use of garlic may increase the risk of bleeding during surgery. While culinary herbs used in small quantities for flavoring are generally safe, consuming large amounts for prolonged periods of time may have a negative effect on the body when going through chemotherapy. 

“With the growth of the Internet, patients have better access to information about alternative products and often turn to dietary and herbal supplements to treat their illness because they think they’re natural and safe,” said June McKoy, MD, geriatrician at Northwestern Memorial Hospital and lead investigator on the ASCO presentation. “What people don’t realize is that supplements are more than just vitamins and can counteract medical therapies if not taken appropriately.”

Dr. McKoy, who is also director of geriatric oncology at the Robert H. Lurie Comprehensive Cancer Center of Northwestern University, said more research is needed to understand which supplements interact with chemotherapy drugs and the extent of those interactions, and encouraged patients to communicate openly with their physicians about the use of supplements. “Patients need to tell their doctors what medications they are taking—including vitamins and supplements—to avoid any possible interaction.”

Recent research shows that 50% of patients undergoing chemotherapy did not tell their doctor they were taking alternative therapies. “Some believe it’s not important, while others are uncomfortable admitting they are pursuing alternative therapies,” said Dr. McKoy. “The truth is, integrative approaches can be beneficial for cancer patients, but it’s important to take these approaches at the right time and under the supervision of your doctor.”

No matter the course of treatment, Dr. McKoy stressed the importance of physicians and patients being more cognizant of this potential interaction and encouraged communication about all herbal supplement intakes. “Patients should bring in labels and bottles to their appointments. This can help the doctor calibrate drug dosage with other supplements in mind in order to prevent toxicities.”