Both patients and healthcare professionals must understand information about prescription drugs to help them use medications in a safe and effective manner. However drug information materials can be difficult to understand — they can be long, detailed, technical, and complex. Comprehension problems can increase the chances that ineffective treatment or medication errors will occur. This paper presents an overview of a large-scale research program on how people understand drug information, especially benefits and risks. It describes basic cognitive principles used to evaluate drug information and shows ways to make it easier to understand and use. Two key concepts underlie this work, cognitive accessibility and alternative representations. They are described and illustrated with sample experiments on comprehension of pharmacy leaflets, TV ads, medication schedules, and side effects.