TORONTO, Aug. 31 (UPI) -- Teens may smoke to "self-medicate" against depression but researchers in Canada say smoking may increase depressive symptoms in some adolescents.

Lead author Michael Chaiton of the Ontario Tobacco Research Unit of the University of Toronto and co-author Jennifer O'Loughlin of the University of Montreal Hospital Research Centre say the study involved 662 high-school teenagers who completed as many as 20 questionnaires from grades 7-11 about their use of cigarettes to affect mood.

"This observational study is one of the few to examine the perceived emotional benefits of smoking among adolescents," Chaiton says in a statement. "Although cigarettes may appear to have self-medicating effects or to improve mood, in the long-term we found teens who started to smoke reported higher depressive symptoms."

Study participants were divided into groups of: teens who never smoked; smokers who did not use cigarettes to self-medicate, improve mood or physical state; and smokers who used cigarettes to self-medicate.

Study participants were asked to rate on a rating scale depressive symptoms such as: felt too tired to do things; had trouble going to sleep or staying asleep; felt unhappy, sad, or depressed; felt hopeless about the future; felt nervous or tense; and worried too much about things.

"Smokers who used cigarettes as mood enhancers had higher risks of elevated depressive symptoms than teens who had never smoked," O'Loughlin says.

The study findings are published in the journal Addictive Behaviors.