Prescribers and patients need information about the established harms of a drug as well as its benefits. This information helps prescribers to calculate the likely balance of benefit to harm before prescribing a drug. It also enables them to avoid using a drug in circumstances of particular risk (for example, renal insufficiency), to choose preventive strategies (for example, using a bisphosphonate to prevent glucocorticoid induced osteoporosis), to know when to monitor for harm when the risk is defined (for example, regular blood counts in patients taking clozapine), and to recognise an adverse reaction when it occurs. Patients need to know about harms for similar reasons. They need to be able to decide whether the likely benefit outweighs the potential harm. They may be more aware than the prescriber of circumstances that prevent their taking the drug. And it is in their interest to be alert to the possibility that an unwanted event that occurs while they are taking the drug is an adverse drug reaction. What is the best way to provide information about drug harms?