A new study has highlighted the risk of social smoking.
It found young people who had the odd cigarette, even just on the weekends, have four times the chance of becoming a full-time smoker by their late 30s than non-smokers.
Researcher Lindsay Robertson says it highlights just how addictive it can be.
"Even if they think they're having one or two cigarettes at the weekend, it actually can turn into a really long-term habit that's really hard to break."
People who smoked occasionally tended to think there was no harm in having a cigarette now and then.