The One Where Rachel Smokes - S5-E18
Corrected entry: At the very end of the show when Chandler is looking for Rachel at her work he takes a drag off of Rachel's bosses cigarette. However we never see him exhale any smoke. He just imitates taking a drag.
The One Where Rachel Smokes - S5-E18
Corrected entry: In the episode where Rachel smokes, she tells her boss that her father is a doctor and he used to tell her 'horror stories' about what smoking does to you. But back in season 2 in the episode where they have two parties for Rachel, Dr. Greene asks Ross to get him a cigarette out of his coat pocket.
Correction: While you can't dispute the fact that Dr. Greene smokes, this doesn't necessarily mean that he would propagate it. He could have told Rachel all the horror stories about smoking while still being addicted to nicotine. Many smokers are very adamant in the belief that smoking is bad.
Correction: He mumbles his words a bit - likely he just didn't exhale much. He definitely takes a proper drag though - you can see the tip glow.