Bias disclosure: I started taking oral probiotics because my doctor told me to and I have vaguely positive feelings about probiotics. I kept taking them because simple inspection with my tongue showed I was developing fewer dental plaques. But the friend I recommended them to wanted actual data, so I did some digging. The results were overwhelmingly positive. They reduced not only cavities, but in the study that checked, total antibiotic usage. In your face, friend who asked for data.
But only one of those tested a probiotic lozenge, the rest were milk products supplemented with Lactobacillus rhamnosus. Not all Lactobacillus species scored so well, so I went to check what my supplement had. Turns out it has no Lactobacillus at all. So I went back to google scholar and checked the bacteria I was actually taking (Bacillus coagulans and Streptococcus salivarius). Luckily the news was still good: in head to head trials, Bacillus coagulans was found to be as effective as a mix of Lactobacillus rhamnosus and Bifidobacterium species, and Streptococcus salivarius also performed well.
Then I found the motherload: someone did a comparative survey. This was less helpful to my cause. Oral probiotics were almost universally found to be helpful to children, but results in adults were mixed. My first argument is “well, yeah, adults develop fewer cavities per unit time than children, so you’d need a bigger sample over a longer time period to detect a difference.” But the studies looked at intermediate results like “bad” bacteria presence, and even the 15 month trial in older and elderly people didn’t see a difference.
My conclusion is that oral probiotics are definitely good for children, and in light of the additional data for my personally, good for me, but possibly not for all adults. I still feel confident recommending other people try them, but not that they stick with them if they don’t see results.
ETA 12/24/19: I’m currently using Florassist (affiliate link).