A sin is if you break a commandment of God. Jesus said ALL the commandments rest in these two:
1. You should love God with all your heart, soul, and mind.
2. you should love your neighbor as you love yourself.
If one truly loves his/her ownself, then that person would not do anything "stupid" to themselves that can harm them. God says our body is His temple and that we are bought with a price. If we do anything wrong to it, we defile God's temple. And that breaks both commandments above.