Background:
E-cigarettes are available in a wide variety of flavors, which increases sensory appeal and stimulates smoking initiation, especially among youth. To determine regulatory measures on flavors in e-cigarettes, e.g. restriction or prohibition, more insight should be obtained in the role of flavors in e-cigarette addiction. Core components of addiction are liking, learning, and wanting. We provide an overview of e-cigarette flavors related to these aspects of addiction, including differences between youth and adults, and smokers and non-smokers. In addition, we aim to identify e-liquid flavors that are representative for different flavor categories (e.g. sweet, fruit, tobacco).

Methods:
A systematic literature review was performed in May 2017 using PubMed and EMBASE databases. Key words included terms to capture concepts associated with e-cigarettes, flavors, liking, learning, and wanting in articles published from database inception to the search date. Results were independently screened (0.92 Cohen's Kappa) and reviewed.

Results:
Searches yielded 387 unique studies of which 32 were included. Research designs varied between cross-sectional, experimental, mixed-method, case study, and longitudinal. Flavors were described as an important reason for e-cigarette initiation. Youth mainly prefer fruit and sweet flavors, while tobacco is more popular among adults. Studies used different flavor categories such as sweet, fruit, tobacco, mint, candy and dessert. E-liquids representing these categories varied across study designs (e.g., vanilla represented the sweet, candy, as well as the dessert category). Nevertheless, results of different studies were comparable for each flavor category.

Conclusions:
Published research mainly focused on flavors for e-cigarette liking. Research gaps exist on the learning and wanting components of addiction. Our review helps researchers developing study designs to investigate e-cigarette addiction. In addition, our overview will provide policy makers a first guideline towards regulating the amount of e-cigarette flavors available in order to prevent e-cigarette initiation among non-smoking youth.