This saccharide is of very minute risk, absolutely, and even more so if compared with glucose, fructose or saccharose.
- Caries is the result of microbiota producing acids which dissolve the enamel, and
- this effect is enhanced when this takes place under plaque when and where saliva cannot dilute the acids
For reason 1:
Most bacteria tested were able to metabolize lactulose with the exception of strains of Streptococcus salivarius, Lactobacillus acidophilus and Lact. fermentum. Streptococcus mutans produced most acid overnight but the initial rate of acid production from lactulose by uninduced cultures was very low. Plaque pH was monitored in 12 volunteers following rinsing the mouth with lactulose, sucrose or sorbitol or Lactulose BP.
These studies in vivo showed both lactulose and Lactulose BP to exhibit low acidogenic potential. Thus, although plaque bacteria are capable of fermenting lactulose, the results suggest that lactulose is likely to pose a small acidogenic challenge to teeth under normal conditions of use.
P.J. Moynihan, S. Ferrier, S. Blomley, W.G. Wright and R.R.B. Russell: "Acid production from lactulose by dental plaque bacteria", Letters in Applied Microbiology 1998, 27, 173–177. DOI
This has to be read in perspective, as S. mutans is capable of using lactulose, but not very efficient in doing so and while overnight the acid production is comparatively higher, it is quite low overall, compared to saccharose.
For reason 2:
Different sugars present the bacteria with different challenges in metabolising them. Dental plaque is mainly hold together by dextranes (extracellullar polysaccharides), which are not easily synthesised and excreted by the microbes, if they are fed lactulose.
Carbohydrate pH extracellullar polysaccharides
--- (mcg/ml n = 2)
Glucose 4.55 110
Fructose 4.8 --
Invertzucker 4.2 --
Saccharose 4.7 1950
Raffinose 4.7 500
Stachyose 4.7 350
Leucrose 4.6 2600
Palatinose 5.1 2070
Lactulose 4.7 --
Formation of extracellular polysaccharides in the presence of selected saccharides by Streptococcus mutans Ingbritt (incubation time 48h / 37°C)
Summary: In the light of recent literature, the authors outline the state of our knowledge of the bio-chemico-microbiological actiology of dental caries. From experimental studies it is evident that the ,,cariogenic" streptococcal strain S. mutans Ingbritt synthetizes in vitro extracellular dextrans suited for plaque formation not only preferentially from saccharose but also from a series of glucose-containing fructosides. No such synthesis was observed with the non-cariogenic S. faecalis. Systematic experiments are indicative of the trend that in the cariogenic S. mutans Ingbritt the activity of the synthetizing dextran sucrase is greater by about one power of ten than in S. faecalis. It seems that the dextran sucrase activity is a contributory determinant of the character of cariogenic streptococci.
A Täufel & K Täufel: "Zum mikrobiellen Verhalten von Gluco-Fructosiden gegenüber den Streptokokken der Mundflora
im Hinblick auf die Zahnkaries", Die Nahrung, 14, 5 1970, p331-337. DOI
Meaning that metabolic activity of cariogenic bacteria is relatively low and their ability to form a biofilm plaque is much reduced if we look at isolated load from lactulose. Since bacteria can adapt and the flora of a mouth change, it's probably still not a good idea to really bath the teeth in lactulose solutions, but given the application as gastrointestinal treatment that's just swallowed quite quickly, the concern seems really low.