Russian spies adept at using sodium fluoride

Excerpt from article titled,

Poison claim in mysterious Surrey death of Russian supergrass

… Former KGB agents have described how Russian spies are adept at using sodium fluoride, an odourless substance that can be lethal in certain doses and, as Mould points out, is complicated by the fact that fluoride is found in drinking water. Russian agents have also been known to administer poisons that can induce heart attacks but do not show up in postmortem examinations…