The underlying mechanism that makes some individuals more susceptible to aphthous ulcers (canker sores) than others is not well understood. There are, however, external factors that have been shown in clinical studies to have a statistically significant impact on the number of canker sores the study participants experienced.
One such factor, with strong clinical evidence linking it to canker sore incidence, is the effect of toothpastes that contain sodium lauryl sulfate (SLS). This is of interest to the typical consumer because virtually all of the leading brands of toothpaste, including the leading baking soda, and the leading "natural" brand, contain SLS.
SLS is a detergent, or surfactant that helps to breakdown and dislodge food and bacteria in the mouth. It also creates foam during brushing.
There have been four studies undertaken to investigate the connection between sodium lauryl sulfate and canker sores. All the canker sore related product-marketing claims found in the literature for various SLS-Free toothpastes are based on one or more of these four studies. Below are the study references, and a brief synopsis of the findings: (Note: the p-values reported in all of these studies refer to the probability that the reduction in number of canker sores was due to "chance", and was not really related to the toothpaste used. Standard practice is to claim statistical significance only if the chance probability is 5% or less.)
Herlosfson BB, Barkvoll P. Sodium lauryl sulfate and recurrent aphthous ulcers. A preliminary trial. Acta Odontol Scand 1994;52:257-59.
In a study prior to this study, it was noted that participants using an enzyme-containing toothpaste, which also happened to be an SLS-free toothpaste, suffered fewer canker sores than a control group using an ordinary (SLS) toothpaste. The enzyme was later ruled out as the reason for the reduction in canker sore incidence. The study referenced here set out to determine if the sodium lauryl sulfate could have been the cause. The results showed a statistically significant decrease in the number of canker sores from an average of 14.3 while using the sodium lauryl sulfate toothpaste to 5.1 canker sores while using a toothpaste without sodium lauryl sulfate. (p < 5%)
Herlosfson BB, Barkvoll P. The effect of two toothpaste detergents on the frequency of recurrent aphthous ulcers. Acta Odontol Scand 1996;54:150-53.
A follow up from the previous study with a larger group of patients. From the abstract: "A significantly higher frequency of aphthous ulcers was demonstrated when patients brushed with an SLS- than with a CAPB-containing or detergent-free placebo paste. An SLS-free toothpaste may thus be recommended for patients with recurrent aphthous ulcers." (The p-values in this study were under 0.2%)
Chahine L Sempson N, Wagoner C. The effect of sodium lauryl sulfate on recurrent aphthous ulcers: a clinical study. Compend Contin Educ Dent 1997;18:1238-40.
Another study confirming the correlation between canker sores and toothpaste (dentifrice) containing SLS. "A statistically significant reduction in recurrent aphthous ulcers was observed during 2 months use of SLS-free dentifrice compared to 2 months use of SLS-containing dentifrice. These results support the results of an earlier independent study, and suggest that use of an SLS-free dentifrice should be considered for individuals suffering from recurrent aphthous ulcers." (p < 1%)
Healy CM, Paterson M, Joyston-Bechal S, Williams DM, Thornhill MH. The effect of a sodium lauryl sulfate-free dentifrice on patients with recurrent oral ulceration. Oral Dis. 1999 Jan;5(1):39-43.
Another study of the same subject, however, though they saw the same trend as the previous studies, that is, the SLS-free toothpaste usage resulted in fewer canker sores than usage of the SLS-containing toothpaste, this study could not claim a statistically significant effect. The most easily quantifiable measure, number of ulcers, had a p-value or probability of chance correlation of 6.6%. Recall that by convention, the p-value must be 5% or less to be considered statistically significant. Given the results of the previous studies, a probability of actual correlation of only 93.4% is not likely to throw much doubt on the connection between SLS and canker sores. It is, however, interesting to note that in the discussion section of this paper, explaining why a larger difference in number of canker sores was not observed, the authors note that the detergent, sodium lauryl sulfate, is an anti-plaque agent with a direct anti-microbial effect. They theorize: "It is possible that any detrimental effect of SLS on the oral mucosa of ROU patients may be offset by this anti-plaque action which may help to reduce secondary bacterial infection of aphthous ulcers. Loss of this action may mask any benefit experienced through a reduced irritant effect with the use of SLS-free dentifrice in ROU patients." Or, in plan English, having no detergent in the toothpaste, as was the case with the control toothpaste in this study, may not be the best solution.