It’s usually what’s called a “lettuce cigarette”. No tobacco in it.

https://www.news18.com/news/buzz/what-do-actors-actually-smoke-in-movies-and-shows-director-explains-4289945.html

Theatre Worker Here:

Most prop masters I've worked with use either this brand or something very similar. From that page the ingredients listed are: Marshmallow Leaves, Red Clover Flowers, Rose Petals, Fruit Juices, Honey. You can also buy packs that are designed to look like real brands(yellow american spirits, the famous "Marlboro Red", and white for marb lights.)

Fake cigarettes have been pretty common props for a while now. Obviously there used to be a lot more actual tobacco use on sets but nowadays, especially with a lot of municipalities outlawing tobacco smoking indoors in public spaces, its almost always all fake/herbal. There are also nicotine free vaporizers that are styled to look like real cigarettes.

They’re herb cigarettes. Think about how long scenes take to shoot, it could be days for a single scene. They’d be smoking multiple packs in a day of nonstop smoking. Even actual smokers wouldn’t enjoy it.

I have a question of the same kind, how do they do when they clearly sniff coke ? What kind of white powder isn't harmful to the nose is used in cinema ?

Just in case someone important reads this:

I really wish movie makers would drop the ‘smoking is {cool,badass,…whatever} trope.’ I’m happy whenever I find that a whole cast never touches a cigarett on camera (think, How I Met Your Mother or The Big Bang Theory)

I've read that they're almost all fake... But I really have no idea 😅

They don't inhale. You can usually tell the real smokers from the ones who don't smoke if you watch the exhale.

If it all comes out in one puff, they aren't smoking, just holding it in their mouth.

If it comes out in a long stream or through thier nose, then they are likely a real smoker.