Herbs have served as medicine throughout human history. Since the passage of the Dietary Supplement Health and Education Act (DSHEA), inconsistent regulatory practices have resulted in widespread, indiscriminate use of herbal supplements. Available data indicate that cancer patients use these products (along with standard treatments) more often than the general population. The reasons cited for such use include improving health, reducing the risk of recurrence, and reducing the side effects of cancer treatments. Herbs, however, contain biologically active compounds and can potentially interact with prescription medications, including chemotherapy drugs. We describe the mechanisms via which these interactions may occur, as divided into pharmacokinetics and pharmacodynamics. We highlight four popular herbs and a medicinal mushroom commonly used by cancer patients-turmeric, green tea, ginger, ashwagandha, and reishi mushroom-along with reports of their interactions with standard drugs. We conclude by emphasizing the need to inform patients and physicians about herb-drug interactions and how to advise patients on appropriate use of herbal supplements to minimize the risk for interactions.