Leaving drug trolleys unlocked and unattended during drug rounds creates opportunities for drug theft and tampering. A new device was developed by our trust to detect when an open drug trolley is left unattended; it then sounds an alarm to remind staff to return to the trolley. This article describes use of the alarm on general hospital wards in one trust in the east of England. When the alarm was installed into drug trolleys on ahospital ward, it reduced the number of times unlocked trolleys were left unattended. The drug trolley alarm successfully changed the behaviour of staff on drug rounds and, in so doing, improved patient safety.