The purpose of this study is to evaluate the impact of concurrent cannabis use and severity on tobacco abstinence. All participants will receive tobacco cessation treatment (varenicline) for 12 weeks. This study will recruit adult tobacco users (ages 18-40) who are motivated to quit smoking cigarettes.

Breath carbon monoxide (CO) can be used as an indicator of recent smoking and is measured by CO monitors. Breath CO can also be used to determine abstinence from smoking. There are several breath CO monitors commercially available, including a next generation model that connects directly to a smartphone and interfaces with a mobile application. Given the importance of comparability between different CO monitors, this study involves a comparison between three monitors to assess their agreement in CO readings and detecting abstinence from smoking.

This is a research study that will test a new remote monitoring technology to assess smoking in the natural environment among adolescents and young adults ages 18-25. After assessment and inclusion in the study, participants will be asked to report on their smoking or vaping patterns by answering questions on a mobile phone for 35 days. Participants will also be asked to make a brief quit attempt lasting for approximately 48 hours. There is no cost to participate and compensation is available to those who qualify.

The purpose of this study is to assess the effects of N-Acetylcysteine (NAC), an over-the-counter antioxidant, to assist adult cigarette smokers with quitting and preventing relapse to smoking. This medication may help people quit smoking by reducing withdrawal symptoms, craving, and preventing relapse, but we do not know if NAC helps smokers early in their quit attempt or after they have already quit.