They idiom, “to take it with a grain of salt,” comes from Pliny’s Naturalis Historia in 77 AD.  He gives a description of an antidote for a certain poison.  Here’s the quote translated from Latin:

“After the defeat of that mighty monarch, Mithridates, Gnaeus Pompeius found in his private cabinet a recipe for an antidote in his own handwriting; it was to the following effect: Take two dried walnuts, two figs, and twenty leaves of rue; pound them all together, with the addition of a grain of salt; if a person takes this mixture fasting, he will be proof against all poisons for that day.”

It suggests that the bad effects can be tempered by taking a grain of salt.

I wish we would say, “take it with twenty leaves of rue,” instead.