AFAIK you can’t overdose b12, but in any case I’d follow a doctor’s advice

“Truth” being a valid defense absolutely means that defamation does not encompass true statements, just as self defense acts are not homicides

No, “truth” is a valid defense as established by the Defamation Act of 2013, section 2

Truth
It is a defence to an action for defamation for the defendant to show that the imputation conveyed by the statement complained of is substantially true.

Same thing, defamation means that the one making the claim is unable to back them up with any evidence