Oncologists face ethical challenges when patients use potentially harmful complementary and alternative medicine in addition to or instead of conventional treatments for their cancer. For example, a patient may forego effective cancer treatment in favor of alternative therapies and suffer significant harm as a result. Similarly, false beliefs about the efficacy of complementary therapies may complicate the process of shared decision making about cancer treatment. In this vignette, we discuss clinicians' obligations and provide recommendations for ethically sound communication practices in this clinical context.