For former smokers, e-cigarettes appear to be less addictive than cigarettes, according to a recent study.

The popularity of e-cigarettes, which typically deliver nicotine, propylene glycol, glycerin and flavorings through inhaled vapor, has increased in the past five years. There are currently more than 400 brands of "e-cigs" available. E-cigs contain far fewer cancer-causing and other toxic substances than cigarettes, however their long-term effects on health and nicotine dependence are unknown.

For the study, researchers from Penn State College of Medicine developed an online survey, including questions designed to assess previous dependence on cigarettes and almost identical questions to assess current dependence on e-cigs. More than 3,500 current users of e-cigs who were ex-cigarette smokers completed the Penn State Cigarette Dependence Index and the Penn State Electronic Cigarette Dependence Index.

Higher nicotine concentration in e-cig liquid, as well as use of advanced second-generation e-cigs, which deliver nicotine more efficiently than earlier "cigalikes," predicted dependence. Consumers who had used e-cigs longer also appeared to be more addicted.

"However, people with all the characteristics of a more dependent e-cig user still had a lower e-cig dependence score than their cigarette dependence score," researcher Jonathan Foulds said in a statement. "We think this is because they're getting less nicotine from the e-cigs than they were getting from cigarettes."

Although many regular users on e-cigarettes are trying to quit smoking, the Food and Drug Administration has not approved them for this use, and they cannot be marketed as a smoking cessation product.

"This is a new class of products that's not yet regulated," Foulds said. "It has the potential to do good and help a lot of people quit, but it also has the potential to do harm. Continuing to smoke and use e-cigarettes may not reduce health risks. Kids who have never smoked might begin nicotine addiction with e-cigs. There's a need for a better understanding of these products.

The findings, which are published in Nicotine & Tobacco Research, also have implications for developing e-cigs for smoking cessation. Foulds believe people might need e-cigarettes that are better at delivering nicotine because that's what's more likely to help people quit.

Previous research shows that nicotine replacement efficacy correlates with higher nicotine dose and faster delivery speed.