Tyramine is a poisonous alkaloid found in the holiday plant mistletoe. While it is useful in some medicinal applications, the berries of the mistletoe plant contain poisonous amounts of tyramine.
In humans, if tyramine metabolism is compromised by the use of monoamine oxidase inhibitors (MAOIs) and foods high in tyramine are ingested, a hypertensive crisis can result. The first signs of this were discovered by a neurologist who noticed his wife, who at the time was on MAOI medication, had severe headaches when eating cheese. For this reason, the crisis is still called the "cheese syndrome", even though other foods can cause the same problem.
A large dietary intake of tyramine can cause an increase in systolic blood pressure of 30 mmHg or more. Dietary tyramine intake has also been associated with migraine in select populations, so if you're suffering from a headache this holiday season, don't blame the kids pleading for an Xbox 360, but instead make sure they haven't been sneaking mistletoe berries into your fruitcake.