Smoking is not a sin as long as its use is moderate enough not to gravely harm one’s health. The Catechism of the Catholic Church states that the abuse of tobacco is forbidden by the virtue of temperance which moderates bodily pleasures, but it does not forbid its use (Catechism 2290). In fact, a number of beatified and canonized saints used tobacco (Blessed Pius IX, St. John XXIII, St. Piergiorgio Frassati), and St. Josemaría Escrivá even asked some of his first priests to smoke so as to be more accessible to the laymen they were directing.
Even so, we have to take seriously the health danger of heavy tobacco consumption that modern medicine has established. What was moderate years ago may no longer be reasonably moderate, given what we know today. In any case, we should avoid the puritanical attitude toward smoking common in today’s society, where practically every form of real sin (and drug use) is promoted, and yet smoking is depicted as intrinsically evil.