This article provides a literature overview of known or suspected interactions of the 15 best-selling herbs in the USA with conventional allopathic therapies for cancer. Herbs with the potential to significantly modulate the activity of drug-metabolising enzymes (notably cytochrome p450 isozymes) and/or the drug transporter P-glycoprotein include Allium sativum (garlic), Ginkgo biloba (ginkgo), Echinacea purpurea (echinacea), Panax ginseng (ginseng), Hypericum perforatum (St John’s wort) and Piper methysticum (kava). All of these products participate in potential pharmacokinetic interactions with anticancer drugs. Healthcare professionals and consumers should be aware of the potential for adverse interactions with these herbs, question their patients on their use of them, especially among patients whose disease is not responding to treatments as expected, and urge patients to avoid herbs that could confound their cancer care.