Doctors are being told not to routinely prescribe aspirin for a common heart condition that increases stroke risk.
Guidelines from the National Institute of Health and Care Excellence (NICE) are set to recommend other drugs instead for patients with an irregular heartbeat, called atrial fibrillation.
Warfarin or similar blood-thinning medicine is best, says NICE in draft advice to be finalised this month.
The advice will affect hundreds of thousands of patients.
But experts say most doctors already follow the advice to prescribe blood-thinners other than aspirin and that the guidelines are "playing catch-up" - this is the first time they will have been updated since they were first issued in 2006.