C. Instrumental variable analysis In the third analysis, the investigators will estimate the effects of smoking cessation therapies on the outcomes using physicians' prescribing preferences as instruments for the prescriptions the GPs issue to their patients. The investigators cannot directly measure the physicians' preferences, therefore the investigators will use the prescriptions they issued to their previous patients as a proxy for their preferences. For example, if the instrument was based on just one previous prescription, physicians who previously prescribed varenicline would categorised as a varenicline prescriber. See Section 2 for further details. As with the investigators' previous studies the investigators will use seven prior prescriptions to improve the strength of the instruments. Using multiple prior prescriptions will maximise power; as with previous studies, the investigators will include multiple prior prescriptions to improve the efficiency of the estimators. The investigators will report risk differences in the outcomes using additive structural mean models estimated via the generalised method of moments.