Additives
Manufacturers of tobacco products deliberately add substances to their products. These make tobacco smoke easier to tolerate and increase the addictive effect.
One study from 2007 indicated that more than 100 of 599 documented cigarette additive substances have pharmacological actions that camouflage the odour of environmental tobacco smoke emitted from cigarettes, enhance or maintain nicotine delivery, could increase the addictiveness of cigarettes, and mask symptoms and illnesses associated with smoking behaviours.
For example, acetaldehyde is formed in high concentrations when cigarette constituents, including sugars, are burned. Animal research conducted by Philip Morris themselves demonstrated a synergistic interaction between nicotine and acetaldehyde: rats pressed a bar more for the combination than for either substance alone. If these data generalise to humans, then smokers would puff more with the combination of nicotine and acetaldehyde.
Sources
2014 Surgeon General's Report: The Health Consequences of Smoking - 50 Years of Progress, www.cdc.gov/tobacco/data_statistics/sgr/50th-anniversary/index.htm.
Action on Smoking and Health (2014), factsheet no:12 What's in a cigarette? www.ash.org.uk/information/facts-and-stats/essential-information.
Deutsches Krebsforschungszentrum (2015) Gesundheitsrisiko Nikotin. Fakten zum Rauchen, Heidelberg, www.dkfz.de/de/tabakkontrolle/Fakten_zum_Rauchen.html.
Mishra A, Chaturvedi P, Datta S, Sinukumar S, Joshi P, Garg A. Harmful effects of nicotine. Indian Journal of Medical and Paediatric Oncology : Official Journal of Indian Society of Medical & Paediatric Oncology 2015;36: 24–31.
World Health Organization. (2015). Fact sheet on ingredients in tobacco products (No. WHO/NMH/PND/15.2). World Health Organization.
Rabinoff, M., Caskey, N., Rissling, A., & Park, C. (2007). Pharmacological and chemical effects of cigarette additives. American Journal of Public Health, 97(11), 1981-1991.
Allen JG, Flanigan SS, LeBlanc M, Vallarino J, MacNaughton P, Stewart JH, et al. Flavoring Chemicals in E-Cigarettes: Diacetyl, 2,3-Pentanedione, and Acetoin in a Sample of 51 Products, Including Fruit-, Candy-, and Cocktail-Flavored E-Cigarettes. Environmental health perspectives 2016;124: 733–9.