**PKTWIN**

Existing data (the fixed-dose NMR and covariates) and DNA samples were selected from a twin registry based study of nicotine metabolism and the impact of genetics and environment on nicotine metabolism (PMID:15527659). Twins from the Registry were recruited into the laboratory study of nicotine metabolism if both twins (monozygotic or dizygotic) were interested and eligible based on a three-stage screening procedure consisting of initial telephone-based screening, secondary in-person screening, and tertiary in-hospital review by the project physician. Exclusion criteria consisted of the following criteria: age <18 years or >65 years; weight >130% ideal height-adjusted weight; pregnancy; use of drug metabolism-altering medications such as anticonvulsants and barbiturates; uncontrolled hypertension; diabetes; history of heart disease; Raynaud's disease; cancer, chronic liver or kidney disease, and/or asthma that was not stable or in remission for at least one year; migraine; anemia; abnormal blood-sugar levels; substance abuse or dependence, excluding nicotine dependence; psychiatric disorders that could limit study compliance or require use of metabolism altering psychotropic medication; HIV⁺ status; Hepatitis B or Hepatitis C; history of vasovagal reactions; discomfort with venipuncture procedures or history of difficult veins.

Verbal consent was obtained from participants prior to collection of possible exclusion criteria, description of study procedures and distribution of a health and smoking history questionnaire (first screen). Written informed consent, which explained study procedures, risks and benefits, was obtained from each co-twin in turn prior to collection of concomitant medications, vital status, and biospecimens (blood and urine). At each of these stages, screening of collected data was performed by a medical doctor to judge whether the co-twin was eligible and medically approved, prior to repeating the process for the co-twin. Prior to the in hospital nicotine infusion, an electrocardiogram, vital status, and a brief physical exam were conducted before final medical approval.

Recruitment incentives include monetary compensation for full participation and the opportunity to participate in the laboratory study portion simultaneously with the participant's co-twin.

**IRP/SMOFAM**

For an integrated research project studying the environmental, genetic and metabolic determinants of tobacco use (PMID:14578134), probands and family members were recruited from those SMOFAM families in which the proband had completed at least seven of the first ten assessments on tobacco use and elected to provide a blood sample for DNA analysis. Probands and each first degree relative completed a family history of tobacco use including sex, age, relationship to proband (biological or nonbiological; full-, half-, or nonbiological sibling), vital status, lifetime “ever” smoking of 100 cigarettes, ever regular use of cigars, pipes, or smokeless tobacco, age at initiation of daily cigarette smoking, average number of cigarettes smoked per day when smoking, ever tried to quit, and success in permanent quitting.

Informed consent was obtained from all participants for each the questionnaire, blood collection, and laboratory study components of the study. Permission to store biospecimens and to conduct additional genetic analyses was obtained. Participants agreed to the provision that genetic test results would not be made available to participants. A United States Department of Health and Human Services certificate of confidentiality was obtained for the study.