If you never smoke, but have before sporadically, and finally get on bc, is it deathly to smoke one cigarette?

Not immediately: Perhaps it might not be immediately deathly to smoke one cigarette, but why even start? Smoking increases the likelihood of blood clots when you're on birth control pills. It only takes one big clot in the lungs, and you're done. And for some people, smoking cigarettes is like eating lay's potato chips: "you can't eat [or smoke] just one.".