The association of waterpipe tobacco smoking with later initiation of cigarette smoking: a systematic review and meta-analysis exploring the gateway theory

INTRODUCTION: There is a concern that waterpipe tobacco smoking (WTS) can lead to the later initiation of cigarette smoking, a concept referred to as the 'gateway theory'. The objective of the study was to systematically review the literature for the association of WTS with later initiation of cigarette smoking. METHODS: We searched Medline, Embase and ISI Web of Science in April 2018 without using any language or date restrictions. We selected …