I don't believe smoking is a sin, but I could be wrong. If you feel it is wrong, even if God would allow it, it will a be a sin for you as your conscious says it is.
"For if because of food your brother
or sister is hurt, you are no longer walking in accordance with love. Do not destroy with your
choice of food that
person for whom Christ died. 16 Therefore do not let what is for you a good thing be [
n]spoken of as evil; 17 for the kingdom of God is not eating and drinking, but righteousness and peace and joy in the Holy Spirit. 18 For the one who serves Christ in this
way is acceptable to God and approved by
other people. 19 So then we pursue the things [
o]which make for peace and the building up of one another. 20 Do not tear down the work of God for the sake of food. All things indeed are clean, but they are evil for the person who eats [
p]and causes offense. 21 It is good not to eat meat or to drink wine, or
to do anything by which your brother
or sister stumbles. 22 The faith which you have, have [
q]as your own conviction before God.
Happy is the one who does not condemn himself in what he approves. 23 But the one who doubts is condemned if he eats, because his eating is not from faith; and whatever is not from faith is sin." Romans 14:15-23 (NASB).