FRANKFURT, June 22 (Reuters) - Germany's Boehringer said an experimental drug to reverse the blood-thinning effect of its stroke prevention pill Pradaxa was shown to be effective based on initial results of a late-stage study.

Pradaxa was first to the mass market of stroke prevention in a new class of anti-clotting pills to replace decades-old warfarin, which requires dose adjustments and dietary restrictions.

But the new class of drugs, such as Bayer and Johnson & Johnson's Xarelto and Bristol Myers-Squibb and Pfizer's Eliquis, posed the danger of internal bleeding if the body fails to decompose the active ingredient, creating the need for reversal agents.

The anti-coagulant effects of Xarelto and Eliquis have been shown to be reversed in late-stage studies by Portola Pharmaceuticals Inc's experimental drug andexanet alfa

Boehringer, which has been developing its own reversal agent since 2009, said on Monday that the drug, idarucizumab, reversed the anticoagulant effect of the Pradaxa blood thinner completely within minutes, according to an interim analysis of a Phase III study.

Cases when patients on blood-thinning pills are in need of an antidote include severe injury or emergency surgery.

In May last year, Boehringer settled claims that Pradaxa had caused severe and fatal bleeding in patients for about $650 million in the U.S.

Pradaxa sales stagnated at 1.2 billion euros ($1.4 billion) in 2014, under pressure from a range of rival products including Xarelto and Eliquis. ($1 = 0.8800 euros) (Reporting by Ludwig Burger; Editing by David Holmes)