Table 1 Eligibility criteria

Inclusion criteria

 Aged over 18 years old

 Smoker of burnt tobacco (≥10 factory-made cigarettes/day or 7 g [0.25 oz]) loose tobacco/day or 14 hand-rolled cigarettes/day)

 Not currently using an e-cigarette, or not having used one for more than 2 days in the last 30 days

 Willing and able to come to the DCRF for the required study visits

 Having a minimum of 16 natural teeth (excluding third molars)

 Being diagnosed with periodontitis, having interproximal pocket probing depths (PPDs) of ≥ 5 mm at ≥ 8 sites

Exclusion criteria

 Having used an e-cigarette for more than 2 days in the last 30 days

 Infectious or systemic diseases (myocardial infarction, cerebrovascular accident; phaeochromocytoma; uncontrolled hyperthyroidism; liver or kidney problems; chronic obstructive pulmonary disease) that may be unduly affected by participation in this study

 Haemodynamically unstable patients hospitalised with severe arrhythmias

 Patients taking the medication adenosine (due to drug interaction risk)

 Lack of capacity to be able to consent to the research project and/or inability to follow study instructions

 Participation in a dental research study within the previous 20 days

 Pregnant by medical history, or nursing

 Received any non-surgical periodontal therapy other than a routine scale and polish in the last 6 months

 Currently undergoing or requiring extensive dental, orthodontic or implant treatment, or treatment for peri-implantitis

Clinical characteristics requiring further discussion with potential participants

 Asthma (severity needed to be assessed, patient made aware that nicotine replacement therapy (NRT) is better than smoking but best to use NRT as a short-term stop smoking treatment)

 Long-term throat disease (severity needs to be assessed, NRT use may exacerbate symptoms)

 Stomach ulcer, duodenal ulcer, irritation or inflammation of the stomach or throat (NRT may exacerbate symptoms)

 Diabetes mellitus (advised to monitor their blood glucose more closely when initiating treatment, advised to discuss this with their doctor or diabetic nurse specialist)

 Those taking theophylline, clozapine and ropinirole medications (metabolised by CYP 1A2 and with a narrow therapeutic window, can be affected by stopping smoking, advised to see their doctor to discuss changing the dose prior to starting the quit attempt).