The automated trial design worked effectively and implemented procedures enabled trial delivery with satisfactory or above satisfactory rates in all key feasibility outcomes other than the saliva sample return rate. This included higher estimated outcome completion rates than expected, three out of four Quit Sense participants installing the app and half of these using it for longer than 1 week. Evidence generated regarding the benefit of increasing incentive payments from £10 to £20 for improving follow-up engagement would suggest that increased incentives would likely improve saliva sample return rates in future work. The trial also demonstrated the feasibility of recruiting exclusively via online advertising for a lower than anticipated cost, although further refinements in advert filtering or consideration of other platforms may be needed to help increase the diversity of the sample.

Based on the abstinence outcome, representing the anticipated primary outcome for a definitive trial, Quit Sense led to a higher rate of biochemically verified abstinence relative to usual care and all end-of-trial abstinence measures favoured the app and represented a clinically meaningful effect estimate, although return rates for saliva samples were imbalanced between arms. Nevertheless, this work adds to a growing body of evidence supporting the efficacy potential of smartphone apps for smoking cessation. How Quit Sense might bring about a change in smoking behaviour is still not well understood, although, for some, learning about their own smoking triggers through the app was felt to be key. This remains an important research question for future studies.

Progression to a definitive trial to assess the effectiveness of Quit Sense, with minor refinements to trial procedures and the app as discussed above, is warranted.