In Infinite Jest, various characters are plagued with teeth-issues, particularly in conjunction with anxiety over their consumption, as teeth are the locus of ingestion, and bare a physical, outward, and socially frowned-upon sign of the decay that comes from addictive behaviors.
Hal Incandenza suffers from potentially psychosomatic toothaches. He also dreams that his teeth fall out, a dream which Freudian interpreted as a fear of castration.
Furthermore, "only the worst kind of naif leaves his toothbrush unattended around E.T.A" (871), because of an incident of somebody putting Betel Nut extract on everybody's toothbrush. The paranoia about toothbrushes reinforces the idea of the teeth as an extremely vulnerable point. Furthermore, has been some speculation that somebody put the DMZ on Hal's toothbrush.
Joelle van Dyne worries about the consequences of her drug-consumption, as she knows it can and will erode her teeth (723-4).
Avril Incandenza goes to the dentist regularly to mitigate the effects of her cigarette smoking, which she deems a "filthy habit." This desire for a clean appearance is also a desire to hide the filthy parts of oneself.
Hal has a similar substance/teeth-cleaning ritual, as he has an elaborate procedure for hiding his pot-smoking involving mouthwash, and also goes "down the hall to take out (his) tobacco and brush (his) teeth..." (864) after chewing tobacco. Throughout, teeth-cleaning serves as a ritualistic purging-type ablution; the act of whitening the teeth leads to a pure external appearance and belies that Darkness within.
Teeth come up in James Incandenza's film "Fun With Teeth" in which "a dentist (Birch) performs sixteen unanesthetized root-canal procedures on an academic (Tolan) he suspects of involvement with his wife." Tolan is probably a reference to Charles Tavis.