Biting people isn't particularly discreet, and a person that gets captured can't dispose of a gland. A similarly deadly poison in the tip of a mundane item that a person might reasonably be carrying seems much more practical, much more likely to succeed, and much more likely to remain undetected

Georgi Markov springs to mind https://en.wikipedia.org/wiki/Georgi_Markov

Poisoned dentures sounds a lot like the fake/apocryphal joke about pens during the space race:

> During the height of the space race in the 1960s, legend has it, NASA scientists realized that pens could not function in space. They needed to figure out another way for the astronauts to write things down. So they spent years and millions of taxpayer dollars to develop a pen that could put ink to paper without gravity. But their crafty Soviet counterparts, so the story goes, simply handed their cosmonauts pencils.

The soviets? They just gave their agents poisoned dentures and called it a day.

Not to spoil a cold war joke about excesses of American procurement, but that story has been debunked pretty thoroughly: https://www.scientificamerican.com/article/fact-or-fiction-n...