Red pills allow programs to detect if their execution environment is a CPU emulator or a virtual machine. They are used by digital rights management systems and by malware authors. In this paper we study the possibility of browser-based red pills, namely red pills implemented as Javascript that runs in the browser and attempts to detect if the browser is running inside a virtual machine. These browser red pills can limit the effectiveness of Web malware scanners: scanners that detect drive-by downloads and other malicious content by crawling the Web using a browser in an emulated environment. We present multiple browser red pills that are robust across browser platforms and emulation technology. We also discuss potential mitigations thatWeb scanners can use to thwart some of these red pills.