St. John's Wort contains hypericin , which inhibits monoamine oxidase, a chemical associated with depression. MAO inhibitors are commonly prescribed by medical doctors to treat depression. Other types of anti-depressants have become more popular because they have far fewer side effects. MAO inhibitors should not be used when a person eats substances containing the amino acid tyramine or bacteria with enzymes that can convert tyrosine to tyramine, viz., alcoholic beverages, products made with yeast, aged cheese, sour cream, liver, canned meats, salami, sausage, pickled herring, eggplant and soy sauce. Otherwise, convulsions, extremely high fever and death by natural causes may occur.
Some plants are lethal even though they are natural.
But if you die from eating a lethal but natural plant, you will not be said to have died of natural causes. Ditto, if you die from being bitten by a poisonous snake whose venom is quite natural. If you die from lung cancer caused by smoking tobacco, a natural plant, you will, however, be said to have died of natural causes. Fleas on dogs are natural. Flea collars are unnatural.