Cigarettes are rolled tobacco wrapped in paper for smoking, sold in packs at any real world corner store. Rockstar has confirmed Cigarettes as one of the consumable items placed through GTA 6, part of the everyday, lived-in world it builds around Jason and Lucia rather than a combat or crafting system. No stats or effects have been attached to them officially, so there is nothing to confirm about how, or whether, they change gameplay. Past Rockstar games have mostly used small items like this as flavor and character detail rather than a mechanic with numbers behind it, and that is the safest way to read Cigarettes for now. Treat this as background texture in the world Rockstar is building rather than a tool with a defined function.
Cigarettes are confirmed to be present in GTA 6 as a consumable, likely picked up or bought at stores like similar items in past games. Rockstar has not detailed exact effects, so we cannot promise any specific benefit. Expect them to add to the world's texture more than the mechanics.
The community companion for Grand Theft Auto VI. Database, maps, guides and tools for Vice City and the state of Leonida.