Thomas, 54, stars alongside Ryan Gosling in the gritty 2013 blockbuster, which features brutal violence and explicit sexual scenes, and the film was classed suitable for over-18s only in Britain and rated R, meaning those under 17 need adult accompaniment, in the US, reported Contactmusic.
Thomas says even she found the film hard to watch at times and was concerned to learn young teenagers could see the movie in France.
"I found (the film) incredibly violent, and I was very disturbed by the fact that in France it was given the official seal of approval that any child over the age of 12 could see it.