The potential multibillion dollar drug is already approved in 33 countries, but the prospects of a green light from US, regulators have been clouded by a trial that appeared to show it performed worse than rival drug Plavix.

Experts have speculated that the lack of effect in US patients might be due to the drug’s interaction with aspirin, which is typically given in higher doses alongside treatments such as Plavix in the United States than in Europe.

The results of an analysis of the interaction between Brilinta and aspirin dosage level, published in US medical journal Circulation, appeared to support that assertion, although it noted that the regional variation could arise from chance alone.

The analysis suggested that US patients on Brilinta also taking low-dose aspirin actually did 27 percent better than those on low-dose aspirin and Plavix.

Deutsche Bank analyst Mark Clark said the research suggested a possible answer for the FDA’s approval conundrum, but he noted that the article stated: “the regional interaction could arise from chance alone,” ie, the analysis is not definitive.

“Thus we continue to believe the FDA has a difficult decision to make,” he said.

The blood thinner is a competitor to Sanofi-Aventis and Bristol-Myers Squibb’s Plavix, which is the world’s second-biggest drug with sales of more than $9 billion a year.

US regulator FDA is due to make a decision on Brilinta by July 20.

AstraZeneca’s shares were up 0.2 percent at 0952 GMT.