The US film ratings board has announced it will now take smoking into account along with sex, violence and adult language when classifying movies.
The Motion Picture Association of America (MPAA) already takes underage smoking into account but will now also consider adult smoking.
Descriptions such as "glamorised smoking" could now accompany ratings.
But anti-smoking campaign group Breathe California said the MPAA plans did not go far enough.
Some critics of smoking in films have said all films showing tobacco use should be given an R rating - meaning only over-17s could see the film unless accompanied by a guardian.
"Clearly, smoking is increasingly an unacceptable behaviour in our society"
He said smoking in films was in decline and three quarters of films that feature even "a fleeting glimpse of smoking" got R ratings for other reasons anyway.
"Clearly, smoking is increasingly an unacceptable behaviour in our society," he said.
The board will consider whether the smoking is pervasive, if it glamorises smoking and if there is an historic or mitigating context.
Joan Graves, who heads the ratings board, said a film such as 2005's Good Night and Good Luck - about chain-smoking journalist Edward R Murrow - would have carried a pervasive smoking warning if it was rated today.
But it would probably have retained its PG rating because of its historical context in the 1950s.
British stance
The MPAA released a statement of support for its plan from John Seffrin, chief executive officer of the American Cancer Society, US Senator Joe Biden and film-maker Rob Reiner, among others.
In the UK, the British Board of Film Classification said it already takes "works which promote or glamorise smoking" into account when classifying films.
Simon Clark of smokers' rights group Forest said there was very little evidence to suggest that children were influenced by watching movies featuring actors smoking.
"Films have to reflect real life," he said. "In most of the western world, a quarter of the adult population smokes and I don't see why films shouldn't represent that, as long as it's not totally gratuitous."
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