The research in The Lancet medical journal compared a new drug called terutroban to aspirin.
Patients with ischemic stroke, a type of stroke caused by a blocked artery, were given one of the two medications. Researchers stopped the study early because the new drug showed no extra benefit.
Previous research in animals suggested it was effective as aspirin in preventing blood clots.
Doctors say any new medication for stroke prevention is going to have to be better before it can replace aspirin.