Data from CPS-II, a major epidemiological study by the American Cancer Society, indicate that smokers who switch from cigarettes to smokeless tobacco have nearly the same disease and mortality rates as smokers who give up tobacco entirely. That is, switchers get almost the same risk reduction as quitters, which is good news for smokers who are worried about the hazards of their habit but are not ready to completely abstain from tobacco. Yet an analysis of the data recently published in the journal Tobacco Control dwells on the negative:
Our study suggests that switching from smoking to using spit tobacco compares unfavourably with both complete tobacco cessation and complete abstinence from all tobacco products…The risks of dying from major tobacco-related diseases were higher among former cigarette smokers who switched to spit tobacco after they stopped smoking than among those who quit using tobacco entirely.
How much higher? The overall mortality rate for switchers was only 8 percent higher, a difference that was barely statistically significant. The differences in disease rates were 46 percent for lung cancer, 24 percent for stroke, and 13 percent for coronary heart disease. Compared to the increases in risk associated with cigarette smoking, these are quite modest, and it's not clear they can be attributed to smokeless tobacco use.
The authors of the report, who work at the American Cancer Society and the U.S. Centers for Disease Control and Prevention, argue that "a causal relationship between the use of spit tobacco and lung cancer risk is biologically plausible given the high nitrosamine content of these products." They cite rodent studies indicating that at least one tobacco-specific nitrosamine is "a strong, systemic lung carcinogen that induces lung tumours independent of its route of administration." (It's worth noting that Swedish productions methods, which are increasingly being adopted by U.S. companies, dramatically reduce nitrosamine levels in smokeless tobacco.) The authors say "it is also biologically plausible that spit tobacco could increase cardiovascular risk, given that these products have been shown to have acute adverse effects on heart rate and blood pressure, and long-term adverse effects on blood pressure and lipid profiles." But the evidence on this score is pretty weak:
Although studies on the incidence of coronary heart disease and stroke have not observed increased risk among men who use spit tobacco, studies on mortality due to cardiovascular disease have observed higher risk among spit-tobacco users, albeit not always significantly higher in smaller studies. Spit tobacco contains numerous chemicals such as nicotine, sodium and liquorice that are known to affect cardiovascular function adversely.
As tobacco control researchers Jonathan Foulds and Lars Ramstrom note in a letter to the journal, the switchers in this study were also more likely than the quitters to develop chronic obstructive lung disease, and it's hard to imagine how using smokeless tobacco could contribute to that condition. It seems more likely that the switchers were different from the quitters in some way that raised their smoking-related disease risks. The researchers controlled for cigarettes consumed per day but not for smoking intensity. If smokers who switched to smokeless tobacco tended to be more strongly attached to nicotine (which seems plausible), they may have taken more puffs per cigarette, inhaled smoke more deeply, and/or held it longer, which could account for the observed differences in disease risk.
In any event, the differences in risk between switchers and quitters pale beside the differences in risk between cigarette smokers and smokeless tobacco users. Estimates from various researchers, based on comparisons of disease rates, indicate that smokeless tobacco is something like 90 percent safer than cigarettes. That advantage would have been clear if this report had included data on smokers who neither quit nor switched but instead kept puffing away. Oddly, the researchers—who work for organizations that are loath to admit there are any differences in risk between different forms of tobacco—did not provide that comparison.