We actually learned about this in med school last week, but I never thought it actually happened to people... Licorice contains glycyrrhetinic acid which inhibits the conversion of cortisol to cortisone. If you have too much cortisol floating around because of this, that cortisol starts activating receptors in your kidneys that are normally activated by aldosterone. This in turn increases your sodium resorption and potassium excretion, so you get high blood pressure and low potassium which can cause arrhythmias. It's called the Syndrome of Apparent Mineralocorticoid Excess and is a usually genetic problem with the enzyme that breaks down cortisol.