Shortly after his death and two inconclusive post-mortems, a former KGB defector told The Guardian about a toxin called sodium fluoride.
I had thought that NaF is used as dental treatment!“The substance is colorless and without smell. It can be applied to personal items — like a pen, phone or door handle — or to places where the target inhales it. It dissolves in the ‘mark’s’ body. It’s undetectable in any post-mortem carried out.”