Non-smoker Alyson, who plays fiancee LILY to Segel's character MARSHALL, was so repulsed by the smell of his breath she vowed not to go near the actor unless he quit.
He says, "She told me she would not do any romantic scenes with me as long as I was smoking.
"And that left me with no other option.
"So I quit the day we started working, and I haven't had a cigarette since.
Segel is now grateful for the decision because he smells sweeter and boasts a higher sex drive.
He adds, "I didn't gain any weight - I was a little nervous about that, but that wasn't a problem.
"I didn't get too grumpy. My clothes don't smell anymore. I really upped my mojo."