It's elemental mercury encapsulated in glass not one of the organic compounds that cause all the acute and longterm health issues. Chlorine bleach is scarier stuff used in the household.

Mercury is mostly banned for being persistent pollutant, next after lead. There is no real safe level of mercury intake due to cumulative damage and hard for body to get rid of.

There is a safe, heck, required level of chlorine intake. (What is table salt?)

Mercury is not any superior at measuring temperature over a resistive electronic or a bimetallic thermometer, or alcohol capillary one. (Except the latter boils and freezes too easily, not a problem for body temperature.)

There was a spate of TikTok's recommending mixing them for cleaning purposes. To the horror of many.

Drink bleach and it'll probably kill you or at least give you terrible injuries.