When does telling the truth detrimental to ones mere existence?

Tweet

The notion of living out the common aclaim of "Honesty is the best policy" has been instilled in humans as a child of every society and cultures internationally; from as far back to the pre-historic ages (I think). However, at which point does telling the truth and striving to be truthful to all, become detrimental to one, ones health, the type of and quality of relations formed over time, and to ones mere existence?