The glycyrrhizin in black licorice lowers potassium levels, and heart rhythm regulation depends on potassium levels. Stop eating glycyrrhizin, and the potassium levels come back to normal.

If you only eat black licorice occasionally, and limit your quantity then so that you do not get enough glycyrrhizin to lower potassium levels to where they actually cause anything bad to happen, you should be perfectly safe. As a precaution, next time you have occasion to have a blood test, get your doctor to include a potassium test. If that doesn't come back close to the low end of the normal range, an occasional bit of licorice should be fine.

If a potassium test isn't too expensive, it might be interesting to have another test shortly after you have consumed some licorice, to try to get an idea of how much it affects potassium for you.

I wonder if the potassium lowering effect of glycyrrhizin could be countered by eating the licorice with something else that is high in potassium. Eat a banana with your licorice, say. Or maybe take a potassium supplement on days you want to eat licorice?