Its the combination of nicotine with certain by-products of burning that lead to withdrawal symptoms
for management and organization purposes
[79] Although several studies have found lower levels of carcinogens in e-cigarette aerosol compared to smoke emitted by traditional cigarettes, the mainstream and second-hand e-cigarette aerosol has been found to contain at least ten chemicals that are on California's Proposition 65 list of chemicals known to cause cancer, birth defects, or other reproductive harm, including acetaldehyde, benzene, cadmium, formaldehyde, isoprene, lead, nickel, nicotine, N -Nitrosonornicotine, and toluene
Elisa Tong of UC Davis
Cigarette smoking and gastrointestinal diseases: the causal relationship and underlying molecular mechanisms (review)